The ruins in Oblivion were of Ayleids, the Wild Elves, native to Cyrodiil.
And the Falmer are what?Natives of Elswhere?
Dwemer were better builders than Falmer so their buildings have lasted far longer and note that Dwemer built mainly underground when Falmer built on the ground where all the weathers cast their affects on them and where the Nords and Giants lives. Also it may be that Nords maybe have used all the well cut stones from Falmer buildings for their own buildings.
The architectural of the Falmer buddings looks as solid as the Dwmer one if not better considering they have shrines that could teleport you.
Also exists the possibility that the Nords destroyed all traces of the Snow Elves they could during the war.
Driving them out with pure force and destroying or as the post above me noted, potentially reusing materials.
That is kinda the only logical explanation, that the Nords took them down stone by stone.