I'm almost certain it's a simple case of limited storage and limiting breakages.

FWIW, I have been known to take my own stemware onboard many times, frankly it's sacrilege to be drinking some of the premium wines offered in many first and a few business class cabins in thimble sized glasses. It's also not uncommon for well-meaning crew to fill those glasses very full, leaving no room for swishing and nose.

BA is far from alone in this, almost all airlines choose their glassware from a 1970s era Trattoria Supplier's Handbook. A few have been daring and gone stemless with Riedel O series, they're far better than thimbles on stems, and they're what I take on board to limit space and breakage. Some traditionalists will innevitably turn their noses up at stemless glasses, I am sure. Certainly at home or dining out I much prefer proper sized stemmed glasses, but Riedel O are a very reasonable compromise.

If you really want a proper Champagne glass, the Riedel XL are my favourites.

First world problems, I know.