DVD

That is correct. But there are other means to find other seasons if one is so inclined.

Officially, yes, the only season released is the first season. But as "proof" mentioned, there are other ways of seeing the whole series. I'm not sure if they're still there, but a few years ago, there were a number of websites offering the "Comlpete Series" on DVD. They were, of course, recorded from TV reruns. But the quality was decent, I know because I have a set myself.

It's worth looking into if you're a die-hard fan of the show.