A friend of mine used to wonder why they just didn't email the darn plans. ;)

The Falcon was "on the lam" after blasting out of Mos Eisely, so perhaps a spaceport free of an Imperial presence was too far away at the time. We also don't know if the Death Star moved on during the time our heroes were aboard.

Leia just saw her whole planet wiped out, so she was probably hell bent on getting the DS plans to the Rebels ASAP, before another heavily populated world was next.

She could also have had doubts about whether she really spilled everything under the mind probe.

Maybe they were betting on the DS coming in without any Imperial Navy support?