Crimea could be cut off from Russia at a few chokepoints at which point Russia soldiers would retreat or run out of supplies. Eastern Ukraine, on the other hand, is connected to the one of the greatest military logistic networks in the world, the Russian rail network.
I am not trying to debate this, I genuinely know nothing about the difference in importance of rail vs air vs whatever else when it comes to military logistics. I found the mentions of the russian rail interesting, so at this point I just want to know more about it in the context of current events.
Long before the Bolsheviks the Russians have seen railway logistics a key to defensive or regional war. Inside their boundaries they can move troops and supplies highly efficiently. It may even help in an invasion of a neighbor that has a compatible rail network that can be commandeered. Rail can move a tremendous volume of stuff at low cost, when they tried to invade Kiev they had to go by road and had a terrible time with sustainment.
The U.S. ability to communicate by air and sea has global reach (why I said ‘one of the best’) but in its range you can’t beat the low cost and high volume of rail.