They aren't. It's not 1860, no one is campaigning on preserving the Union over other policy considerations. While there would be intense disputes about how to treat people who didn't want to be separated from the US, and about disposition of federal property in the event of a divorce, few in NY or CA outside of the Republican minority in each state would expend even the slightest effort on keeping Texas in the Union as such, though they might oppose bad divorce terms on those other issues.
On the other hand, all the other Republican-led states would oppose the idea fiercely, because it would be a giant shift away from the Republican Party in the national balance of political power. (And, yes, Texas is far from solid red by population, but that's not what matters.)