Because Apple is about integration and experience. It's the only platform I know of outside of Blackberry (and Blackberry is dead, so it doesn't even count), where every message is synced across your Computer, iPhone, iPod, and iPad. Why fragment this? It creates confusion.
Not only, but iMessage is much easier to use than text messages. I don't even pay for unlimited text messages like I used to, because after iMessage came out, I went from ~2000-3000 texts a month to under 500 texts a month, since everyone I message uses iMessage or has Verizon, which offers free mobile to mobile texting anyways.
Also, as far as the US goes, all for major carriers offer unlimited texting. In fact, unless you're on prepaid, I'm pretty sure it's your only choice.
Also again, iMessages are kbs in size, maybe a few MB at most.
iMessage is just Kik, or Whatsapp, or BBM, or AIM, or whatever other service you can think of, but just integrated at a system level.
Also I can't think of any plan that gives 250mb of data that offers the iPhone, except for prepaid plans which are extremely expensive compared to their competitors.