SSL and TLS seem to be somewhat interchangeable. So much so that they are generally lumped together ("SSL/TLS") when referring to HTTPS and other services. Its almost as if TLS is version 4.0 of SSL. Why wasn't named that instead of coming up with a different name?
I believe this is mostly due to the fact that SSL was never a considered an internet standard.
This quote is from the SSL 3.0 RFC.
When TLS was accepted as an internet standard, the people in charge probably wanted a new term to distinguish it from the older, "non-standard" SSL protocol.