Why aren’t UMP clones as prevalent as MP5 clones?

So from what I've heard, the UMP was supposed to be a cheaper alternative to the MP5 that was made of polymer. It seems like there are tons of companies making the MP5 now (Zenith, Century, and PTR for example) but very few making UMPs. Is this because tooling is not as common for the UMP? What's the reason?