Why on earth are all these William Castle films so hard to get on DVD?

I'm a horror fan and am trying to get around to these films, but it seems like most of Castle's titles are rare and/or ridiculously expensive to obtain on DVD. I paid $40 for a used copy of "I Saw What You Did" (which, given how much people are asking for it, was a really reasonable deal), and had shell out $25 for a used copy of this film. Luckily I adored "I Saw What You Did" and am happy to own it, but what's the deal with his movies being so hard to come by?