It's my understanding they prefer releasing the stories individually as it allows them to give each one singular attention where a season box set you'd have the less popular ones being overlooked. Plus season sets just wouldn't be a practical approach to Hartnell and Troughton.
However, that point is about to become redundant, with only two more individual releases to come, unless there's more recoveries or they animate up Crusade.
And season sets are surprisingly possible for half of Hartnell and Troughton, thanks to the animation-completed stories...
Season One: Only Marco Polo's missing, and the narrative gap is at least covered by the telesnap precis.
Season Two: Animate up Crusade 2&4 and that season's 'complete'.
Season Six: Is already 'narratively' complete thanks to the Invasion animations, in that Space Pirates is a rare 60s example of a totally skipable story - not a verdict on its quality, it's just that there's no lead-in from/to the surrounding stories, no characters/monsters from old stories, or brought back later (unless you reckon that the Colony in Space IMC used to be the Issigri Mining Corp, as the New Adventures suggest); there's not even the usual throwaway line about it in the next story ("Och, Doctor, the mud in these tranches is even worse than in the tunnels on Ta").
So all three of those could be put out as season sets, with suitably worded cover copy. And even Season Five would hold up surprisingly well now if they just animated up the missing episodes of Wheel in Space (seeing as Wheel 1 would explain, if only in passing, Victoria's departure). I could see that happening, by the way, as it would also mean every Cyberman story 'existed' 'complete'.
