Hill Street Blues - only two seasons have been released, and this was a bloomin' LANDMARK series which changed the face of TV. WTF?
Sales are the big killer for most of these sets. As was mentioned upthread,
NYPD Blue had its first two seasons released in six-disc boxed sets with fancy packaging that sold for around $60; this was in the waning years of the show. When they didn't sell well, there were no more sets for a year or two, then the third and fourth seasons came out, priced at $40, but with four double-sided discs in those slim-line cases. Even
those didn't sell well, probably in part because the show is a.) old news, and b.) no longer in reruns here in the U.S., so there's been nothing since then. Really drives me up the wall, because my two favorite episodes in the whole series are in the fifth season.
Anyway,
HSB ran into a similar thing; it's been out of the public eye forever, and for whatever reason, people weren't biting.