>Why do those CNT-kevlar sheets have to be replaced every 4 month?
Kevlar breaks down in high temperatures and loses a portion of its 'toughness', and as part of the composite it'd be sitting in a metal box under direct sunlight for most of the day when the tank is deployed to 80% of the world. Primarily the sandbox, where most 'warfare' tended to happen in the last half-century.
This isn't to say that the armor suddenly becomes worthless after 4 months, merely that the armor is potentially degraded.
>And why is the difference in price is so astronomical between those and composite CNT armour?
At present only one
factory in the entire world can make sheets of Carbon Nanotubes, and the process is fairly complicated
to make an attempt for understatement of the year, one of the multiple reasons why copycat mass-factories haven't sprung up in places such as China.
Bog standard Carbon Fiber sheets, however, can be (and are) mass produced for relative cheapness.