We DID try to squeeze in as much flavor as we could, but two things limited us...
1) We wanted to have a one-monster-per-page (or close to it) format, so that navigating the book would be MUCH easier.
2) We wanted to keep the book world-neutral. There's no mention of Golarion at all in the Bestairy. It's actually kind of tough to get super detailed in the flavor text without assuming too much about the world in which the monster will appear!
In the end, I think that for the vast majority of the monsters, we managed to get a LOT more flavor text in there than most versions of the Monster Manual. There are, of course, some exceptions (the succubus and the linnorms come to mind), but overall I'm pretty happy with the amount of flavor text we DID manage to get in there and retain both of the 2 points above.
And in any event, we've already got a book line that does lots of flavor for monsters (the Monsters Revisited line). We didn't want to have too much reprinting of material. And since the Bestiary's primary job is to provide the raw rules for the monsters, that wasn't really a concern.
Right. I understand all that. I would still buy the book sight unseen as I already did. And I'm pretty happy with it overall.
And I have those- monster revisited items. And their great. But if I were reviewing the book, thats the bad I see. Its not really a bad thing with keeping point 1 and 2 above in mind, but if someone went in thinking it was going to be a Paizo product that had lots of fluff as many of the other products, they would be disappointed.