I have a theory:
Maybe Arcsystem works is busy doing other shit, and... well, the GGAC port wasn't that much of a priority for now.
See Arcana Heart 3. Game has a faster pace than Brahzbloo and it uses the same netcode, but so far it's the game with the best performance of them all (above Extend and P4A). Maybe they did focus a lot more, since AH3 was originally intended to become a retail title.
Since GGAC was planned as a digital game, maybe they didn't test or adjust the netcode enough coz they are busy with Chrono Phantasma and other stuff.
But well, at least it seems like they will fix this netcode... in a future, according to some things I read here.