The 8 custom character comes from the display controller itself as it only as ram for storing 8 of then.
Ah yes, that too 
I just took the question on a more basic level. As in, why there isn't more RAM to have more than 8 custom characters in the first place? Which I think is due to it being an old standard when RAM was bulkier, more expensive or something like that. Still it could be interesting to know how old the HD44780 standard is. Googling/yahooing showed up nothing.