1. Russia, being too entrenched in Ukraine, wanted some global focus shifted away from Ukraine, and managed to get Hamas (probably via Iran) to attack Israel last October. This would predictably become a very hot issue - especially in the US - and make the world "forget" Ukraine. There's been a lot of writing that the support for the Ukraine war will only last as long as there's mainstream focus on it, and that no new wars pop up.

2. The Gaza plan failed. Israel attacks Hezbollah and Iran, which means neither of those can help Syria. The retaliation from Israel was too strong.

3. Vital support from Russia, Iran, Hezbollah is now severely limited.

And that's pretty much that. The rebels in Syria saw their opportunity, and seized it.

What's more, is that Syria has long been an important supply route for Iran and Hezbollah. With rebel control, which is anti-Iran and anti-Hezbollah, this will have much wider-reaching impact.