Well, you are assuming that they are actually have a logic and implement it.
Very likely they - in good faith BUT often very wrongly - believe that once a version has been assigned the "long term" badge, it means that it will be "long term", while in reality it will be soon be replaced by a NEW long term version (for one reason or the other).
In other words, it is difficult to make predictions, expecially about the future.
Specifically :
V7.20.7 [long-term] is released! - #239 by jaclaz
They simply betted on the wrong horse.
In any case, we are already on 7.21.4:
V7.20.8 [long-term] is released! - #65 by jaclaz