Hmmm...
WG started building matches using a "tank weight" formula, which led to lopsided matches. WG markets and sell some tanks with preferential MM. Consumer buys preferential MM tanks so they can grind crews and earn credits faster, without having to be in the original +/- 2 or 3 MM.
WG hires some guy who was a Russian CC as "Director of Game Balance". Said balancer, developers and WG management see they are not making money on their free to play game, so introduce some OP premium tanks. Those tanks make money either from the sale, or the poor schmoes who now have to obtain premium shells to defeat said tanks. WG decides they are not getting enough revenue to pay for their yachts, junkets and dancing girls, so they balance the game by introducing 3-5-7 MM as the primary balance tool. Said Director of Balancing is apparently a tool, too. Players say this MM is garbage, so I will use my Pref MM tanks to avoid their stupid game mechanics to some degree.
WG now sees these pref MM tanks as the root of all evil, after demoting said Developer of Balance, and decide to "change" the offending Pref MM tanks so more people will spend more money to compensate for the what will undoubtedly be seen by the players as another botched balance attempt.
WG's balance attempt is just another attempt to generate revenue, either from increased credit costs for the premium ammo you will need to spam to be competitive, or the money you will need to pay for a different premium tank.
Thank you WG for breaking an initially engaging game and pooping on your customer base.