why interdimensional beings?

They were Lucas's way of punkin Spielberg into doing aliens

Lucas always wanted aliens, he wanted them in it since at least mid 90s. But Spielberg was done with the alien topic and clearly said: NO ALIENS! As they only wanted to make a new movie when all three agree, this was one of the reasons, why we never got an Indiana Jones in the 90s. Lucas always came back with the idea and always got rejected, like a running gag. In the end Lucas got the idea with them not being aliens, but interdimensional beings. Spielberg agreed finally, more likely because of getting over it as him being totally into the "what beings are they".

Here in Spielbergs own words, it's crazy they put this officially on the DVD:
https://www.youtube.com/watch?v=rE7fzr6lQ-s

The check cleared, which likely was Spielberg's main concern. They look like aliens, so just call them something else and get the film out.

Lucas wanted to bring in the supernatural for the even numbered movies in this franchise, which was this one and Temple of Doom. The next movie will be the 6th installment, so we can expect to again have some supernatural force in it

I personally liked there being aliens since this was set in the 1950s when alien stories were popular.