Text messages pre-date all these fancy 'data networks.' They work by hiding the text data within the voice transmission signal.
Basically, the phone companies discovered they had a little bit of spare space in the signal and invented a way to use it. As such, they put in what they could fit. It's why texts are limited to 160 characters.
So the idea of tripling the amount of possible characters (regular, bold, and italic) was probably not high on their list of priorities.
Now, Apple could currently add such things to iMessage, but I'm sure they don't want to make it confusing. It would be kind of annoying to format texts and have your iPhone friends see it all but your Android friends not. So I guess iMessage is inheriting the limitations of SMS just so they can live together happily.