I believe it's due to the shrinking of the player base resulting from awful decisions by WG. MM can't keep up with a dwindling player base and as a result this is what we get at Tier 10:
6 or 7 high alpha TDs per side.
Too many Chieftains and 279E's.
3 artillery per side.
3-4 light tanks per side.
Add to that awful maps that take years to fix and virtually no impediment for unskilled players to get to Tier 10 (except time) and they've created a combination of awful things that makes Tier 10 virtually unplayable. The traditional heavy or medium tank has become absolutely worthless and your 46% win rate allies will jump in to save you only after you die.
Where should WG go from here? They think slashing 10% off of premium tanks is a solution. They thought giving the worst class in the game Stun was a smart idea. Flooding tiers with bots doesn't address the cause of the problem. It seems like they're doing nothing right in the moment and follow up with good decisions about 2-3 years too late.