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Beyond MAD guaranteeing peace between great powers, these had good reasons to not initiate landgrab wars.
When a country annexes a piece of land, unless they can ethnically cleanse the area they'll end up with a bunch of foreigners who might not want to belong to that country. This can mean a bunch of uncomfortable things. One of the main thing is that they have to keep the place occupied at all times and spend a bunch of money and time to it.
The US is also confined on her continent, she could only get overseas territories which sucks to reach. The Soviet Union had also population problems to do such moves albeit it had long dry border.
The US found the solution around 1906 when they figured out what to do with Cuba and the Philippines which they held occupied: they turned them into clients.
The SU acted the same with the Eastern European countries, the Eastern Block was the Soviet Union and her clients.
The rationale is: we don't have to control the whole population if they can be controlled via a regime we control. The US runs her massive client empire super cheap. The Book says: in the fiscal year of 2004 they spent a bit more than 1% of the budget on the routine maintenance of 81 client states. And clients are there to project and amplify the power of their patron.
All great powers and even lesser powers moved to this system.

Why Russia does the landgrab now? Because she failed to acquire Ukraine as a client for the west just outbid them when they bought up all the corrupt politicians and because the clay she incorporates is populated by the same people who lived in the Soviet Union. There is no ethnic, cultural, economical or even political problems. Nada.