Probably because those of us who actually introduce people to the game and such find the idea of trying to change the game to something more akin to 3e as sort of offensive and infuriating. If OSRIC is to be an AD&D, it should be that, not some hybrid fantasy heartbreaker.
Replacing Descending AC and THAC0 with Ascending AC and BAB is a
far cry from making it some kind of hybrid game; it is a far less significant change than replacing the Attack Matrix and Weapon versus Armor Table with THAC0 in the first place.
Replacing WP and NWP with skills and feats would be a hybrid game. Giving AD&D classes AEDU powers would be a hybrid. Ditching race/class restrictions and allowing pick-a-mix multiclassing would be a hybrid.
BAB versus THAC0 isn't a significant mechanical change; it's a shibboleth for Edition Warriors.
I know many favor WotC D&D on this site and have no problems with making it closer to WoTC D&D than AD&D, but that doesn't mean that everyone who plays AD&D is in favor of that. The old schoolers may be the minority these days, but that does not make us irrelevant. We may be unpopular with the WotC D&D forms of players these days, but when they hijack our games, it sort of miffs off some of us.
I'm a TSR guy. I'm an AD&D guy, though... I find myself wishing AD&D were a lot more like
BECMI than Pathfinder. There are a lot of changes I wish I could make to AD&D, but making it more like any version of WotC D&D-- except
technically PO and DF-- aren't among them.
And I certainly don't have any patience for people whining about THAC0.
But BAB isn't more popular than THAC0 because the OSR is being overrun by disenfranchised 3.X fans. It's more popular than THAC0 because it's just common sense-- it is just a very little bit easier and more intuitive, without changing the gameplay or the system math one
iota. Regardless of which side of the "debate" you're on, it's a pointless and silly hill to die on and being
this antagonistic over it is just... childish... and toxic... and
shameful.