Because text creation is served by very very good programs.

such as livetype, aftereffects and photoshop...

http://www.kenstone.net/fcp_homepage...damentals.html

You can create simple or complex text in all these programs, and animate it in a way that would be impossible to do with a few dmx channels.

Peoples text needs are complex, and cant easily be crammed into a few dmx channels.

Livetype is bundled with Final Cut Pro:
http://www.apple.com/finalcutpro/livetype.html