There is still no explanation why the Romulans couldn't save themselves. They must have known years before that their sun was going to explode. The Federation at least had enough time to build tons star ships. The Romulans should have, too. Not to mention they had tons of space ships already. I don't get why they write the Romulans in need of rescuing when they are not planet bound primitives.
If the series use proper science for supernovas now, meaning they only threaten their own solar system and it was definitely the Romulan sun going kaput, it does make even less sense that Romulans needed rescuing. They could have just transported their population on Romulus over time to some of their colonies.
Relocating billions of people is not exactly a piece of cake. And as Federation was supposed to 'only' evacuate 900 million, it seems Romulans themselves must have evacuated majority of the population.
As for why didn't the begin sooner, I'll repost what I said in another thread:
Also, about 'without the warning,'we now a have a reason to suspect that Betelgeuse will go supernova soon. That 'soon' may mean tomorrow or it may mean in hundred thousand years. So even with better tech it might not be exact science. The Romulan scientists may have indeed known that the star will explode within next couple of millennia, and the empire may still have not taken necessary precautions. You don't need to look far to see how that sort of insanity could happen. Supernova talk gets branded as leftist fear mongering, and the Praetor promises to 'make the Romulus great again' instead of spending immeasurable resources and upend peoples lives to prepare for a calamity that probably doesn't happen during his lifetime. And people buy that, as the truth would be too uncomfortable and accepting it would mean making serious changes to their lifestyles.