First, it's considered vital by Russian strategists so the likelihood of Russia using a nuclear weapon if Crimea is at stake is higher than for anything else in the conflict.
Secondly, historically, the claim of Ukraine to Crimea is far more tenuous than for the other area at stake. Crimea has been at odd with the Ukrainian federation regarding its independence for as long as it has existed as an entity independent of the USSR. It was rattached to Ukraine in the act acting its existence as a political entity and one of its first move was declaring independence in 1992 which Ukraine rejected. They were de facto independent and very pro-Russia until 1995 when Ukraine sent its army and forced their government into exile. It makes Crimea pretty unique in the conflict as there is a legitimate claim that it actually wants to be a part of Russia rather than Ukraine which is why the western reaction following the 2014 invasion was so muted.