everyone says they want more bp, but the bp limit likely comes directly from paper mario 64 (one of the inspirations of block tales)
while yes, it would be cool to have more bp, the bp limit is likely a homage to paper mario 64 where the max bp in that game was 30. most people probably don't even realize that this is a pm64 reference though
everyone loves bp but not giving it a limit (like paper mario the thousand year door) would make the game too easy since you could easily equip every defense+, safe guard, charge defense, etc. and still have bp left over for almost every other card
having a limit actually proposes a small challenge for the player, needing you to be strategic with what you pick for specific fights.
if you're saying "just increase the limit" they could very well do that, but i think they wouldn't because paper mario is a big inspiration for the game and they'd likely want to reference it
edit: i forgot to say that the bp limit is also likely for balance