It's all a matter of choice, some people like races that respect the DF canon (MDF doesn't), total conversions, or even just a single script or a single workshop added on top of vanilla DF.
If/when they manage to make a working "mod manager", all those small mods could in theory get a big boost in popularity, from getting a similar "easy to use" manager that is able to keep mods and scripts up to date, and customize your experience in a few clicks.
The problem for most modded games is that there is no standards (file names, .dif when adding/removing from a file, structure in the zip file, versioning...), so making such a tool is hard, and particularly unrewarding, because most modders would not agree to stick to rules that a single person decided to try to impose.
For some platforms, it is a bit different, by having a set of tags and a search system, to at least make searching and filtering easier (steam's workshop, the Nexus sites, among others).
Those sites are not perfect, and it relies on modders not checking every tag for every mod just to advertize, and popular mods will usually still be at the top, but it is able to push forward a lot more "similar" content, to be able to quickly compare and decide.
All DF has is a forum, and any non official website would have a hard time convincing modders to use it, making it take a lot more time to search through mods and find what you were looking for.