There's the real strategic concern that escalating too quickly will have nuclear repercussions. But the deeper reasons are visible if you view most governments as military industrial corporations stacked under a trenchcoat. The true motivator is that the longer the war continues, the more money will flow from their respective tax payers into their pockets. They don't care about Ukrainian lives, they don't care about Russian lives. The popular support for the war and lack of domestic casualties means they get to ply their trade of death, and they come out smelling like roses. Opposing Russian colonialism is a noble cause, but the nobility belongs to those who are dying in the foxholes, not the warmongers who are squeezing this crisis to get more capital.

Western leaders don't want Ukraine to win. They want Russia to lose. A quick cauterized wound is less damaging than a slow bleed out. Total bankruptcy of the Russian war machine is the objective, the economic elimination of their primary trade competitor.