There is several reasons but probably the biggest one is

It creates a 2 class community and therefore divides the userbase. (look at MK8 right now)

Disregarding your idea, not commenting about if it is bad or good, but WOW are your prices horrendous.

edit: also we have no idea on all the stages in the game so far.  There may be quite of few of these brawl/melee stages you want already in the game.

But again, wow horrible prices you posted.  

the content of the game is good enough, i dont think it need a DLC.

Stages as DLC would be a waste especially since we have a Stage Builder now. I would prefer characters as DLC, bring on the characters!

Bayonetta DLC would be awesome. They could even phone it in and just have Bayonetta as an alt for ZSS.

Edit: They could phone it if they couldn't add Bayonetta as an extra character for ESRB reasons or something is what I meant.