Mainly durability, comfort, precision and speed.
Modern MBT and their fully stablizied main armament guarantee first hit shots at 50+ km/h in any terrain. Given the top speed of a WWII tank was less than 40km/h and most tanks lumbered to battle at something like 10km/h and had to use shotting stops the requirements for the whole drive unit has greatly changed.
And the reason why the best tanks look similar and use a simliar travelling gear means it´s the best one around.
Which thing is more expensive?
You know how tracks are propelled and steered. And you know how wheels are propelled and steered. All MBT torque rods to keep their wheels elastic and absorb impact. So you got like 7 wheels on each side, and a huge giant cogwheel on your backside to power the tracks.
Now, replace that system with an equally agile and reliable wheel system. You need a couple of axles, fixed and steerable and you need to redo your drive chain. You need a total new system as you can´t use anything out of your old one.
It needs to be equally durable, agile and good as your track kit. Means expensive. And totally useless. Noone installs 2 perfect systems on 1 machine for the same purpose. If it´s war, you ride to battle, no matter the distance. No matter wether you´ve got rubber coated tracks mounted or steel ones attached.
However, out of a logicistical aspect, transport by train or LKW is far cheaper in terms of diesel or gas and you don´t suffere tear and wear of a road march on your tanks.
The current steel tracks on modern MBT are quite durable - a couple of thousand km, closer to 10 than 1. So you don´t have to change them that often. The rubber blocks attached to it to save wear and tear on the track and the asphalt of streets though need to changed more often - but that´s only a few hours work of maintenance for a single crew team.
So in short - replacing/adding a very reliable, durable and awesome way to move 50+ tons in all terrain and conditions with a totally different system to move said 50+ tons on a street to save wear and tear on tracks is useless, too expensive, too fragile, too complicated and thus doomed and causing epic fail if you try it.
It´s not needed. Otherwise, if it was feasible and awesome, everyone had it. But - nooone does.
If you still can´t get my point - I lack the proper english fullily support my cause - go and a have a look at some modern tanks. Try to look for technical charts as well. Then look at your car and imagine what you´d have to do if you wanted it to run on simliar tracks like those tanks feature. Then reason about price.
