The first two seasons being available streaming likely helps keep their DVD price down. Third season's unavailability, I believe (but of course don't know for sure), must be due to rights disputes.

Yes, I'm also glad to have my DVDs on my shelf from their first availability, for under $80. ;-)

I also have them all streaming, because it remains accessible to those who bought it in that format back when. I feel lucky with that. I came very close to missing that opportunity, not knowing the window would close.