Now, I don't know anything about the aliens' society and political system. In human history however, the death of a souvereign ruler, for one thing, ending a campaign is not unheard of.
Alien mindset doesn't explain it -- not to me, anyway. They were out for conquest, with a warfleet and ground troops, meaning they had to have previous experience at large-scale fighting -- meaning, in turn, that they should have known to expect losses. As well, they knew we had a certain level of technology, and although our modernized energy weapons were ineffective, they should also have expected less-advanced weaponry (projectiles) to be brought into play on our part. Having already blown up a great deal of our stuff, it would seem sensible to expect them to continue, modifying their tactics as needed -- after all, they also had colony ships with them.
Now, I don't know anything about the aliens' society and political system. In human history however, the death of a souvereign ruler, for one thing, ending a campaign is not unheard of.
It's also been known to piss off the defenders even more . . .
The stack of improbabilities here is just
way too tall.