There are a number of reasons why a studio would be reluctant to release their shows on DVD. In some cases, it may be due to disputes over ownership rights to certain properties, such as music or characters (like the soundtrack to Daria or the use of popular tunes in Alvin and the Chipmunks or the use of stock movie and TV show footage in Jim Henson's Muppet Babies or Tiny Toons boasting episodes featuring caricatures of deceased celebrities like Ronald Reagan or Princess Diana). Or it could be that the studio is busy compiling a decent set of extras and bonus features. Or it could be due to contractual obligations. Or it could be that the network which owns said show may not want to release it on DVD right away for fear that the viewings of said show on TV will fall off (provided the show in question is still airing on TV, that is). Or it could be that a prior DVD release didn't sell well and thus the company may be reluctant to release another DVD set for fear of losing money (like how the initial DVD releases of Hanna-Barbera's Huckleberry Hound or Comedy Central's Drawn Together apparently didn't sell well enough to warrant any follow-up releases). Or a studio could consider producing DVD releases of some of their other properties (such as live-action movies or TV shows) to be a higher priority than their cartoons. There are a number of factors involved, depending on the show or studio in question.