What are they hoping to accomplish? Wouldn't a surrender be better for both sides? Then the Rebels might be allowed to live and the Empire would have fewer casualties to report to a potentially sympathetic Galactic Senate (that had not yet been disbanded).
This falsely assumes the Imperials won't just kill (and potentially torture) them immediately.
After capturing them, Vader tells the Imperial commander:
Leave that to me.
Send a distress
signal and then inform the senate
that all aboard were killed!
It's pretty clear that at this point, whether they fought or simply surrendered the outcome is the same: death (or disappearing, which is probably worse) at the hand of the Imperials.
And this is ignoring the potential of Vader finding the Death Star plans on board. From a plot perspective, obviously he does not (and cannot). But Leia can't know what Lucas has in store for her, and if you had that information on board, would you have tried to surrender? It'd be suicide.
Also, note that Vader is very convinced they had the plans in how he interrogates the rebel officer:
Where are those transmissions you
intercepted?
It's likely that the Rebels understood why the Imperials were pursuing them and trying to capture the ship intact (since they did in fact intercept plans).
Keep in mind an Imperial class star destroyer could very easily have destroyed the blockade runner they were on board. The rebels could have surrendered there, too, instead of fighting.
Which makes the whole thing pretty understandable, they knew they had information that they could not destroy since it was vital to their cause. They knew that information would doom their entire crew to a painful death. They knew the imperials wanted it. The might as well continue the fight as best they can, even if it's hopeless.
