UM lacks visibility
On many sites related to Ubuntu the last articles or updates on UM do date at least 2-4 years... All is into Mint, Zorin, POP!, etc.
Nothing to say about UM ? How to attract more users then ?
Silence is often the road to oblivion.
Just thinking
W
I have noticed a bit of slowdown lately, too, ever since Martin left Canonical. His focus on making UM visible on social media really made a difference.
Maybe they are not reporting on UM, because for them it is only the old one boring thing that just works... 
So nothing to be excited about or nothing to complain about...
Just saying
W
Maybe you can do more. When I read tech sites comparing those other Ubuntu based OS's I often comment how similar they all are with one exception, Ubuntu Mate has the best forum. Look at Mint, pages and pages of unanswered questions. (I am a member there too). Everyone gets an answer here, and some of the best Linux experts around ( cudos to Pavlos and Richard).
You can help too, talk Ubuntu Mate up on Linux and tech sites that allow comments.