Heh... at the end of the day, I think the entire Large PC concern comes down entirely to Large PCs expecting/wanting to use Large weapons. Everything else is a middling concern.
Being able to go nutso in combat is one of the biggest things there is for a good percentage of the message board population. That's why the Sharpshooter / Great Weapon Master feats are still argued about on and on about how "required" they are, and how "unbalanced" they are etc. etc. etc. We now throw in the possibility of Large PCs... ones that by the rules in the MM as they currently stand use large weapons that do much more damage... we now open up a whole can of worms where all of a sudden the entirety of the game's "balance" is thrown off because of the disparity in damage capability.
And it's not like they could even just remove the "large weapons" ability for Large PC creatures and replace it with the bonus d4 from like the Enlarge spell. I mean hell, look at all the complaints about making the centaur into a Medium creature! If *that* is getting some players all worked up... that's NOTHING compared to the tempest of complaints that would occur if WotC allowed for the existance of Large PCs but then removed their ability to wield oversized weapons as part of the PC write-up.
So WotC is (probably rightly) just not even opening that can of worms. No Large PCs. Easy, simple, and they never have to engage with the crapstorm from combat-first players who want Large PCs so they can be even stronger in combat, and who would go ballistic if they didn't get to because large weapons were restricted to them.