A big part is because last week was a very easy m+ week, everyone has burnt themselves out a bit and this week, and likely next a lot less people will be running m+, after it should go back to normal.
Also, CoS 11 on teeming fortified skittish.. lel. Not a fun one bl.
Also NH came out, so all the top players who are usually all over that stuff are doing normal/heroics/mythic raid in their downtime and want a break.
Aswell as a lot of people recently hittin 55.
Claims above that people don't do it cause the gear in it isn't worth it anymore is lul worthy. Considering a lot of BiS trinkets, relics, and 2pc items are all from m+.