To answer the question in the headline:

1. I find the GitLab UI to be cleaner in general and easier to find my way around. However, this is purely a matter of taste and probably not a strong reason to move.
2. I also like how GitLab is open source. I am far from an open source zealot, but I do prefer to write and use open source software. While Github is full of Open Source projects, Github itself is proprietary. In contrast, Gitlab has a well-supported open source version.
3. The project import feature worked very well, so it was trivially easy to move the code, branches and issues over.

The author goes on to describe why GitLab’s project management workflow works well for him.