Part of the reason is that people are still finding out about it, Project has no marketing so it grows organically, in the last year the number of contributors grew by 25 percent.
Another problem is that it still needs polish in term of ease of use, for example it takes me forever to search for packages using the nix-env command but using the website it takes less then a second, That’s a basic feature that still does not work correct, Plus their documentation is still not great in my opinion, I actually helped improved it and the improvement they made is still not really good IMO.