They actually explained it awhile back in a live QnA
TJ- Its based off experimentation. When we started off initially when we got versions of the servers removing tanks and replacing them just like PC did, then we changed to removing them but making them premium for those that had them, but still taking out of the game. This meant that players new to the game would be like ‘’But I want a chance to get that tank”, so in this case we went with just adding the new takes to that tech tree, because the others are still very viable tanks and it’d let players still earn them because they’re legendary
Because F&%# balance. I imagine TJ typed that by rolling his head back and forth on his keyboard; I'm sure other WG decisions are made in a similar fashion.