It's a combo of not wanting to get killed and perhaps hoping to benefit from Russia's immense natural resources if the two countries merge*. Until Ukrainians show they are willing to fight and die in large numbers, the West will be largely hands-off. It took 100K dead on both sides in Syria within a year before Uncle Sam started making noises about supplying the rebels. A year after hostilities commenced, the death toll in the Ukrainian conflict is under 10K. This is more a shoving match rather than an all-out knock-down conflict.
* Without oil and gas, Russia's GDP per capita would be pretty close to Ukraine's.
Ukrainian politician Tatiana Montyan has opinion that mainstream of Ukrainian events is oligarchic clans raiding each other assets. War always was reliable way to solve somebody’s financial problems.
Old Russian saying is when lords are fighting haircut of serfs suffers.