Just speaking hypothetically now ...
If the government wanted to keep ET/UFO information secret (which we know they do and have done) and if they have some sort of ongoing interaction with ETs of some sort (which has been rumored for years and there is some evidence of) then mightn't they want to push this interdimensional idea to throw us off the track? It totally lets them off the hook, you know?
"UFOs are from ,,, uh ,,, another dimension. Yeah, that's it! So, there's ... uh... no point in digging into it any further. There's nothing to see here, folks."