Again, there are other factors that play into why other season sets of certain TV shows are delayed for a long period of time.
The first three seasons of Rocky and Bullwinkle were released within a year of each other, then there was a delay of several years for the remaining two seasons, and one of the main reasons was because so much of Sony Wonder's distribution library shifting.
Similarly, the first two Old School sets of SST sold really, really well, and were released within a year of each other, but likewise, there was a delay of several years before we got Vol. 3, and there were many factors at play, and like with Rocky and Bullwinkle, the shift from Sony Wonder was one of those factors.
The first two seasons of ED, EDD N EDDY were released with a generous amount of special features at the height of the show's popularity, and there's yet to be any further seasons released, but this shouldn't come as a surprise, considering these are distributed by Warner Bros. (since they own Cartoon Network), and we know how bad they are about their releases.
The first three seasons of GREEN ACRES were released by MGM over a decade ago, and finally over a decade later, Shout! is releasing the last three seasons.
It sucks, but this is just how it goes sometimes. I think we're lucky enough to get the first three seasons of TMS as it is, because TBH, I really can't think of many other Disney shows or Disney-owned/controled shows that have gotten proper DVD releases - despite Disney having its own brand of DVD. I mean, I would love it if RECESS got season sets, and I'm sure people like Drtooth, mr3urious, and others would kill to have even older Disney Afternoon shows on DVD, but I don't look for any of this to happen anytime soon.