The simplicity of it helps. Starting from scratch, here is what you have to do to post something on Twitter and RSS:

Twitter:
1. Goto twitter.com
2. Create an account
3. Post
4. Tell friends your twitter ID

RSS:
1. Research various web hosting companies
2. Sign up for one web host, by paying a monthly fee
3. Research RSS programs
4. Decide on an RSS program
5. Write something and post it
6. Tell friends a complicated URL to follow, and also help them understand what RSS is and that they need a program

Accessibility is a big thing, especially when talking about a product or service for the mass market. Twitter is a consumer service, not just "RSS with an SMS gateway". It lets people tell others what they are up to, in a quick and efficient way.