Some are just plain better, as you state. Even if they were each best at something, diversity comes at a very high cost. Every additional design of MBT means

Some are just plain better, as you state. Even if they were each best at something, diversity comes at a very high cost. Every additional design of MBT means an additional supply chain for parts and ammunition. Additional personnel are needed for every line of MBT. Since you have a different logistics line for each tank, they can't support each other on the battlefield without bringing their whole logistics with them.