Why is northern Eriador (Lake Evendim, Fornost) deserted at the end of the 3rd Age?

[Inziladun]

Originally Posted by Snowdog

So, anybody have any theories on the abandonment of Annuminas?

I think it was a gradual abandonment, with a final blow coming when the ruling seat was moved to Fornost. They were in need of a fortress as opposed to the picturesque city by the lakeshore. I doubt it would have been in one deciding move to abandon a thriving city.

I think it was a gradual happening, occurring for the reason you give, and also because many of the Dúnedain of Arthedain had perished in the long-term fighting. With so many gone, perhaps Annumínas was already falling into decay, even as Minas Tirith was later observed to be when Pippin went there with Gandalf.
I see another parallel with the abandonment of Osgiliath, the location of the King's House in Gondor, and the removal thereof to Minas Anor at about the same time.