Well, Web UI TADS needs you to be running the game on a dedicated server, using a seemingly-modified version of the language. It’s not like you can make a Web UI game and have it work in Parchment, and not a lot of people have a spare server to host things on for themselves.
Also working on your game and iterating on it in Web UI is an absolute nightmare. Someone was posting here about it once, and he had to upload the game to the server, wait for that version to go live, then test it from his personal computer as a client. He was asking if there was a way to test it locally on his machine, but I don’t remember any answers coming of it. I’m guessing you would need to set up a server on your coding rig, and have the computer basically connect to itself, and restart the server and reconnect every time you need to test it.
EDIT: Also also, I remember there being a discussion around the problems of needing a game to be hosted on a specific server while submitting it to a comp, instead of just sending a game file.
EDIT 2: More info on why we don’t see much of Web UI anymore can be found here.