Because Blackberries are aimed at the corporate market. A company will pay a pretty stiff amount (roughly $30k for 500 seats) in order to have BlackBerry Enterprise Server which integrates with their Exchange server (or whatever is supported/used by the company). On top of that, the companies would buy a allocated amount of shared minutes across all the phones within the organisation (again, for the 500 phones example, this would be around £8000 per month) from the operator.

In other words: Blackberries are a highly profitable business. Implementing something which is as low bandwidth as text messaging isn't an expensive project. Plus, non-corporate users often don't have access to the same services, or pay for them as part of their contract.