Two reasons:
First of all, The arabs are totally against US forces on arab land and it would jeoperdize the whole deal. Shiates in general hate Americans almost as much as they hate Israeli's.
Second reason is that France wants to lead this force and the though the US is fine with it, they're forces will not accept a french comand.
Israel and the US have a mutual defence agreement... (though i don't think the US needs us to defend it.. hehe)... so I guess having American troops in Lebanon just won't fly.
For the record, I'm against any foriegn trrops in Lebanon.