Why did they bother doing the whole "Send everyone to another time period" thing as a method of evacuation instead of just...well, getting in ships and evacuating? If they were advanced enough to make a time portal, they could make rocket ships...
We can't assume that, since we're never given any information about the Sarpeidons' spaceflight capability. Technical advancement in one area doesn't necessarily mean the same level of achievement in other areas. It's like asking, "If we can send men to the moon, why can't we cure cancer?"
Exactly. Sarpeidon seems to have put its energies and ingenuity into exploring time, not space. And maybe there are no other M-class planets within light-years of Sarpeidon? Piling into rocket ships does you no good if there's nowhere to go . . ..
(In my book, if you'll indulge me, I speculate that Sarpeidon never developed space travel because there are no other planets in their solar system, so they never had any incentive to develop a space program. They would've had to skip the baby steps and go straight to warp capacity if they wanted to get anywhere.)
As to why they didn't ask their neighbors for help, who's to say they had any space-faring neighbors in the vicinity or were even aware that other civilizations existed?
Remember, the Enterprise
is way out on the final frontier. It's not like Sarpedion was sitting right next door to the Federation or something. Or regularly engaging in trade and diplomacy with the Andorians or Romulans or whomever. Sarpeidon was off by itself in the middle of nowhere, busy exploring its own past.
(Imagine that we suddenly discovered that our sun was about to go bang. It's not like we can fire off an SOS to some friendly aliens, asking for help.)