Hey brandon - I think it's the same reason governments had such a hard time regulating synthetic cannabis. Every time they declared one, another would pop up to take it's place. With crypto, it's the reverse - they're popping up like mushrooms, but at this stage, it's almost impossible to tell which ones are deadly.
See, in order for your idea to work, the exchange would have to be prepared to purchase your coins for fiat then and there. By funneling it toward one coin, the exchange takes only 1 risk (that bitcoin might drop). If they offered it for every coin, they would take the same risk for every currency offered. 
Further, there is probably massive concerns regarding money laundering, the legitimacy of alt-coins as a whole etc etc. The regulations are murky - policies differ. Legally speaking, it's a cluster-fuck. Everyone wants to get rich, but few are willing to risk the noose in order to get there.