Well originally its because old painters and mosaicists could not into perspective

And so this started a tradition that carried on for a while even after artists learned about perspective and vanishing points, primarily because artists wanted to make sure you could see everyone's face.

In a lot of those, Judas is the only one on the other side, to show that he is the asshole.

But that being said, there are a lot that show everyone sitting around a table properly.

Looking deeper at that Byzantine Mosaic, I found that it is hilarious.

The guy that Jesus is hugging looks extraordinarily uncomfortable, while the guy next to him seems to be regretting his life choices.