Might seem silly, but the choice just came from looking at (https://code.dlang.org/search?q=websockets) and seeing that serverino is the only one maintained. vibe-d isn't listed there, I only learned about that later. Basically, serverino has good SEO.
Well, you might have searched for GitHub or got there from some page where serverino was mentioned (forum, ...) or maybe someone suggested it to you in the community. Once a program of mine was mentioned in a telegram channel and I started seeing a lot of stars on github and I didn't understand why.