ITX I believe has some PCI lane things which I don't understand, but really it's just about personal preference.
With a single GPU and 2 sticks of RAM you can have an amazing PC (7700k, GTX 1080ti, 32GB RAM anyone?) in a tiny case, but some people prefer a larger case for many reasons, and the likelihood is you'll just be running the same kit too.
My opinion? Find a case you like the look and size of then build a system into it.