It is simply easier this way?
Don't get me wrong, it probably would be nice to be able to continue in ufoai2.3 with the old savegames. But how much work would it be to translate the savegames of a 2 year old version to this new one? Lets say, the structure of theese savegames would be quite similar, many concepts have changed. There are no medics hireable anymore, for example. So every hired medic would to be eliminated. If I am not wrong, the capacity of the quarters have reduced from 50 to 20. So there is the possibility there are savegames with more personal, than quarters avialble. In my savegame of 2.3 I found out that one can build more than one aircraft in one hangar. In 2.3 this is not possible anymore. What to do, if you find a base with 5 aircrafts but only one hangar? And the alien hangar? Theese are not part of the base anymore but are build on the surface seperately. Only one alien hangar can be build per base and there is no distinction anymore between small and large hangar. What to do of a base consisting of 5 workstations and 5 greater alien hangar? Only delete the alien hangar? Build them on a random spot on the surface neglecting the cap you would have, if you have startet your game using 2.3 in the first place? And if you just delete the hangars, what about the captured ships in them? There is the possibility one already has captured, developed and dismantled alien ships and does not possess a single hangar anymore.
And if in the 2.4 version of UfoAI the restriction is implemented that all base rooms have to be connected, what to do then, if removing an alien hangar seperates your base into two seperate parts? Just randomly rearrange the rooms?
It probably will be possible to transform your old savegames into the new version. But to do so, it requires manpower to write routines that judge all theese questions. And even if it would be done you would end up with a transformed savegame that doesn't really reflect your progress anymore, since you would have chosen different options if you'd played with 2.3 rules right from the start on.