There are more reasons to that than AI preferences or A/B/C bonuses but also the fact that it (along with great lighthouse which is the earliest) taken faster than terracotta. It's favored by tradition empires which currently have much stronger early games, and more science to research them, more culture to unlock them, and more production and population in the capital to build them. So the game mechanism itself tweaks it like that.
I like this comment of yours, and I had to think about it for a bit.
I mean obviously the word *favoured* is important here. That is - for Terracotta Army for example, I can say that that's favoured by Authority because it synergizes with Authority's other bonuses.
My worry though is that Great Library is generally too inaccessible with its culture demand of 4 policies and becomes like a pseudo-policy-tree-completion wonder for Tradition. What do I mean by that? I mean basically all the other Wonders, except possibly Hanging Gardens but not even, can essentially be built by any policy tree as soon as you get the tech for it, because the culture from any tree will match up pretty well with the tech pace. Great Library is out of sync with that pattern. Moreover, I can think of perfectly good reasons for lots of different wonders to be built by different contenders. For example...
1. Terracotta Army
a. Authority - Synthesizes with bonuses.
b. Progress - Good for later expansion and deprives others of a bonus.
c. Tradition - That extra military count is important.
2. Temple of Artemis
a. Authority - It works if you're tending towards producing ranged units.
b. Progress - General growth in a large empire.
c. Tradition - Helps the Capital even more.
Statue of Zeus I might avoid as Tradition though, whereas I'd veer away from Parthenon as Authority other than to deprive other players...though I'd probably not be on that side of the tree with Authority anyways. The point is that realistically for nearly all of the Ancient/Classical Wonders, there's at least 2 policy trees, if not all three, that can meaningfully access the wonder and build it with useful, constructive consequences. Great Library seems to fall outside this realm: it's not practically accessible to all trees and feels like it sides towards Tradition - but this is what I expect of a policy finisher, not a normal wonder. So yeah...I don't like it. Dropping the Policies by 1 would make GL far more interesting.