I think the reason is SMS compatibility.
No reason the lowest common denominator must dominate.
Look at these cats. Teh Lol! http://bit.ly/zzu3
then that message is meaningless without the url right? (in as much as it had any meaning at all)The simple Twitter 140 limit doesn't map to that at all, and coupled with the fact that Twitter SMS is disabled in a good chunk of countries, it seems silly to think of it as an SMS publishing system.