Because SMS is terrible as a text standard, and people want to use a rich messanger. SMS was used when there wasn't much alternative as Nokias didn't have great app support. People already knew of better things and used ICQ, MSN Messager et al. even without being technically inclined. The limitation was the platform.

It's also why iMessage hasn't taken off. We are used to things working cross platform, the iMessage lock-in doesn't interest people as it isn't providing much convenience for its limitations.