It's just much simpler if all that's required is to pick a single metamagic in a drop-down list. Also easier to deal with, AI-wise.
Though, you can have several metamagic effects if your character is a Cleric with a power that provides automatic metamagic effects.
'Say there is a chunk of meat. Pirates will have a banquet and eat it! But heroes will share it with other people. I want all the meat!!' - Luffy in One Piece