Chieftain

Ectar:   In a word - Technology.   If you see an MBT and what World of Tanks players would call a "heavy" side by side, the MBT is pretty much the same size.  Over time we discovered better armour composites and pretty much everyone is using the same or similar guns so they know what to expect will be shot at them. Battlefields have changed and there is no ongoing war that dictates certain requirements in vehicles. In a similar vein combat doctrines have changed over time too. You don't need a "breakthrough" tank or an "infantry support tank" any more. It's the same with modern jet fighters compared to old piston engines fighters.  Look at planes like the Spitfire, Mustang and Ilyushin IL-2 and now look at modern day equivalents such as the the Typhoon JSF the F-35Lightning/JSF and the  A-10 Warthog (I know it's not modern per-say but look at it's role).   Vehicles and combat styles change with technology.  The progression to MBT "jack of all trades" style was necessary due to the role of the tank in modern battlefields. The original Mk 1 tanks were used to cross trenches and they were armed with machine guns only till someone figured out if you put a big gun on one you can take out other tanks.  Bi plane pilots used to wave at each other, then they'd throw stuff at each other, then they'd use handguns, then someone mounted a machine gun and the rest as they say is history.