150 pages in the music forum (one of which is closing in on 3000 posts). Tens of thousands of musicians, bands, artists to choose from with a large variety of styles and new releases coming out weekly, with music that can be seen as far back as 100's of years ago. Not to mention many of which have landed of fair or better amount of success in secular and/or Christian circles. And since most people listen to music on a daily basis and buy new music regularly, and since it can easily be shared on the site, it makes for a busy forum.
Compared to Christian movies, which have a few decades of history, most of what is considered the best are decades old. The modern movies are usually not very good and mostly barely even break into the mainstream. Christian movies are just not very popular. Which is why the threads die.
If a topic doesn't have enough support to maintain it, it won't be put in. Which is 99% of ideas for new room/forums.
Also the site owner tries to keep the number of section small so it's easy to navigate. Some sites have so many options it overwhelms some. So filling it with rarely used niche areas just bloats the site.