if enough people demand it, unless there is some rights issue, it often comes out. sometimes they go the manufactured on demand (dvd-r) route, and for my money i'd rather stick with netflix streaming (in HD as applicable) than overpay for a burned disc with no extras and iffy player compatibility. it sucks, but when you can get a whole season of a show on blu-ray with extras for under $10, you don't want to pay $20-40 for a MOD joint in standard def without any remastering.

tvshowsondvd.com is probably still a good site for gauging fan demand.