"Why do you like this so" is a short way to say "why do you like this so much". So, the phrases are a little different in meaning. The second, with the "so", emphasizes that the person liked it very much and you want to know why they had such a powerful liking.

The difference between "Why did you do like this" and "Why did you do like so" is that the latter phrase is a older style of speaking. They both mean the same, but "Why did you do like this" is the more normal way to say that phrase.