The crew of the Nostromo were guided by fear that if they simply abandoned ship that the Xenomorph would successfully infect any crew that came to salvage the Nostromo and that Weyland-Yutani would continue with their plan take it to Earth where it might cause an outbreak.
Ash (in the novelisation) lays it out quite nicely;
'I cannot say. There is a distinct possibility the alien will
successfully infect the boarding party and any others it comes in
contact with before they realize the magnitude of the danger it
presents and can take steps to combat it. By then it may be too late.
'Thousands of years of effort have not enabled man to eradicate other
parasites. He has never before encountered one this advanced. Try to
imagine several billion mosquitoes functioning in intelligent consort
with one another. Would mankind have a chance?
'Of course, if I am present and functional when the Nostromo arrives,
I can inform the boarding party of what they may expect and how to
proceed safely against it. By destroying me, you risk loosing a
terrible plague on mankind.'
Alien: Official Novelisation
Although Ripley's reasoning isn't laid out on the final cut of the film, we do get an idea of her motivation from the original "Starbeast" version of the script. In short, the alien needs to die.
HUNTER: Blow the ship up?
ROBY: And the creature with it. We can make it back to Earth in the lifeboat.