Why didn’t Abu Bakr bring the people together to pray Taraweeh in congregation, whereas ‘Umar did (may Allah be pleased with them both)?

Firstly: 

An-Nawawi
(may Allah have mercy on him) said: Taraweeh is Sunnah according to
scholarly consensus.

End quote
from Sharh al-Muhadhdhab, 3/526 

Al-Khateeb
ash-Sharbeeni (may Allah have mercy on him) said: They (the scholars) are
unanimously agreed that it is Sunnah and that it is what is referred to in
the words of the Prophet (blessings and peace of Allah be upon him):
“Whoever prays qiyaam in Ramadan out of faith and seeking reward with Allah,
his previous and later sins will be forgiven.”

Narrated
by al-Bukhaari.

End quote
from Mughni al-Muhtaaj, 1/460. For information on its being
prescribed to offer it in congregation, please see the answer to question
no. 21740. 

Secondly: 

With
regard to ‘Umar (may Allah be pleased with him) being the one who first
brought the people together to pray Taraweeh in congregation, whereas Abu
Bakr (may Allah be pleased with them) did not do so, this question was
answered by Imam ash-Shaatibi (may Allah have mercy on him) in his book
al-I‘tisaam, 1/194, in which he said: Abu Bakr (may Allah be pleased with
him) did not do that for one of two reasons:

1.

either
because he thought that the people’s praying qiyaam at the end of the night,
and what they were doing, was better in his opinion than bringing them
together to pray behind an imam at the beginning of the night; this was
mentioned by at-Tartooshi;

2.

or it was
because he (may Allah be pleased with him) had little time to examine such
minor issues, when he was focused on dealing with the apostates and other
issues that were more important than Taraweeh prayer. Then when Islam became
well-established at the time of ‘Umar (may Allah be pleased with him), and
he saw the people scattered in the mosque, as it says in the report, he
[‘Umar] said: If I bring the people together to pray behind one reciter,
that will be better. When he did that, he pointed out that their praying
qiyaam at the end of the night was better. Then the salaf (early
generations) agreed that this was valid and they approved of it, and the
ummah cannot unanimously agree on misguidance. The scholars stated that
consensus can only be on the basis of shar‘i evidence. End quote. 

And Allah
knows best.