Why ukraine didn't take all of Donetsk & Luhansk since 2014

I'm sure it's not for lack of effort or desire.

Zelenskyy thinks he's in a position to make threats because Russia has so far failed to win and he's receiving vast support from the West

Good point. They were warring more subtly.

They didn’t have the support from the West.

Because making war against Russia isn't bungy jumping. You're making a commitment to a lot of your people getting killed and infrastructure being destroyed