Believe it or not, there are some TV shows that haven’t been released on DVD. 
Only Season One of Malcolm has been released, and that was back in 2002. It doesn’t look like any others will see release anytime soon, whether because of the expense of music licensing or whatever.
Similarly absent from DVD is Eddie Murphy’s “claymation” series The PJs.
If anyone knows about any preservations of these I’d be curious to know more. Thanks.