Why has the civilisation of the Muslims fallen behind others?

It is not fair for anyone who wants to examine the causes of
any nation’s decline to focus only on some outward aspects of its decline
that everyone can see, because achieving cultural advancement is a huge
process in which innumerable interrelated factors play a role; these factors
are accumulated throughout the years and penetrate deeply in all aspects of
life, which leads to raising the level of communities little by little.
Humans can hardly feel this progress because it is like the light of dawn;
it keeps coming out until its light spreads over the horizon. 

The Holy Qur’an tells us some of the aspects of material
advancement that were – and still are – a means of trials and tests, of
which no one understands the great wisdom behind it and the great impact it
has on historical dynamics except Allah, may He be glorified and exalted. 

Allah, may He be glorified and exalted, says (interpretation
of the meaning):

“On the Day when those who disbelieve (in the Oneness of
Allah Islamic Monotheism) will be exposed to the Fire (it will be said):
“You received your good things in the life of the world, and you took your
pleasure therein. Now this Day you shall be recompensed with a torment of
humiliation, because you were arrogant in the land without a right, and
because you used to rebel and disobey (Allah)”

[al-Ahqaaf 46:20]

“Do they not travel in the land, and see what was the end
of those before them? They were superior to them in strength, and they
tilled the earth and populated it in greater numbers than these (pagans)
have done, and there came to them their Messengers with clear proofs.
Surely, Allah wronged them not, but they used to wrong themselves.

Then evil was the end of those who did evil, because they
belied the Ayat (proofs, evidences, verses, lessons, signs, revelations,
Messengers, etc.) of Allah and made mock of them.”

[ar-Room 30:9-10]. 

Al-‘Allaamah Muhammad Rasheed Rida (may Allah have mercy on
him) said:

The first Muslims’ belief in the Oneness of Allah (Tawheed),
their knowledge of Him, their love for Him and their reliance on Him are
what purified their souls, honed their ambitions and gave them a sense of
pride and power; that enabled them to establish truth and justice, and
motivated them to conquer lands, rule nations, ransom them from spiritual
and intellectual slavery to soothsayers, rabbis, monks and clergy, liberate
them from the injustice and tyranny of kings, establish the foundations of
civilisation, and revive and develop different branches of science and
technology. They were able to achieve all of that in an unprecedented manner
that no other nation on earth had ever achieved or even come close. Gustave
le Bon, the famous sociologist, said: Mastery of different branches of
science and technology was not possible for any rising nation in less than
three generations: the first generation clings to the past; the second
generation started the process of freeing themselves from blind following;
and the third generation is the generation of independence and hard work.
(He said that this applies to all nations) except the Arabs; they are the
only ones who were able to lay the foundation of science and technology in
the first generation. 

I say: The cause of that was the teachings of the Qur’an,
which taught them to be independent in thinking and to avoid blind
imitation. It taught them to be prepared to lead mankind in spiritual and
worldly matters alike. But all of that disappeared for subsequent
generations, after the demise of the Islamic caliphate and after Arabic
civilisation ceased to flourish, when power shifted to the non-Arabs who had
nothing of Islam except superficial traditions and were not strongly
connected to the guidance of the Qur’an.

Tafseer al-Manaar, 11/173 

We cannot ignore the impact of conspiracies, both internal
and external, against our ummah throughout the ages, which led to its
enemies getting ahead of it in material matters. But our thought and the
thought of our ummah is far above feelings of defeatism, relying on others
in order to achieve revival, and feeling overwhelmed in the face of great
responsibility, the responsibility of being in a leading position in
relation to other nations. As for our enemies among the Israelites, they
excelled at making the most of every opportunity and they strove hard in
engineering events that had a global impact. They strove persistently for
decades until they achieved what they wanted and reached a position of power
in our times. But that has nothing to do with the verse (interpretation of
the meaning): “O Children of Israel! Remember My
Favour which I bestowed upon you and that I preferred you to the Alameen
(mankind and jinns)” [al-Baqarah 2:47], because
this verse speaks of previous blessings that Allah had granted to the
Children of Israel throughout history, and is meant to remind them of that.
Among these blessings are what is mentioned in the Holy Qur’an, where Allah,
may He be glorified and exalted, says (interpretation of the meaning):
“And (remember) when Moosa (Moses) said to his people: ‘O my people!
Remember the Favour of Allah to you, when He made Prophets among you, made
you kings, and gave you what He had not given to any other among the Alameen
(mankind and jinns, in the past)’” [al-Maa’idah 5:20]. 

Al-Haafiz Ibn Katheer (may Allah have mercy on him) said: 

Here Allah, may He be exalted, reminds them of previous
blessings that He bestowed upon their fathers and forebears, and the favour
He bestowed upon them by sending the Messengers from among them, revealing
the Books to them and favouring them above all other nations at that time.
End quote.

Tafseer al-Qur’an al-‘Azeem,
1/255 

However, we regard our current situation with optimism that
motivates us to advance further and succeed by striving harder to use all
possible means, both spiritual and worldly. The foundation of that is the
righteous individual who always remembers that Allah, may He be glorified
and exalted, is watching him in all situations. Such individuals are a
constructive factor in the reform of communities and families; they do not
wait for the revival to be presented to them on a golden platter or hope
that programs will be set up for them to achieve that. Rather they hasten to
take the initiative and strive hard to do all that is good and beneficial,
putting their trust in Allah in the hope that He will bless their deeds, no
matter how small, and reward those who do good for their good deeds. Allah,
may He be exalted, knows who is trying to spread mischief and who is
striving to do good, and the followers of His Messengers will prevail and be
victorious if they take the measures that lead to victory and advancement.
Allah, may He be glorified and exalted, says (interpretation of the
meaning):

“Indeed, they planned their plot, and their plot was with
Allah, though their plot was a great (one, still) it would never be able to
remove the mountains (real mountains or the Islamic law) from their places
(as it is of no importance).

So think not that Allah will fail to keep His Promise to
His Messengers. Certainly, Allah is All-Mighty, – All-Able of Retribution”

[Ibraaheem 14:46-47]. 

And Allah knows best.