I think there is a fan theory that Romulan space is pretty small (relatively speaking), which is how they're able to defend it well with a small number of ships. Which also explains why they're not able to have enough ships to evacuate Romulus.
I think people tend to underestimate what's required to evacuate a planetary population. That's billions of people! The 24th century we see might have some massive passenger transport that could hold, say, 10000 passengers. You'd need hundreds of thousands of ships that size to evacuate the whole planet. And the logistics would be a nightmare's nightmare. A ship that size probably doesn't make planetfall. How do you get them loaded? Think about how long it takes to get a widebody jet boarded - maybe 500 people at most. What about the quadriplegics? What about the bedridden? What about the pets? Etc. etc. etc.
True. Unfortunately, holding people in the transport buffer is one of those things you have to entirely forget exists if you want to tell certain stories. The transporter in general could trivialise a lot of story beats.