Question

why does plasma last longer than whole blood

Answer 1

In a vial if you centrifuge it at sufficiant RPM & G force you'd seperate the plasma and the serum.
The plasma itself does not have any coagulating factors therefore it can last much longer before it's not analyzable.
While the serum has these coagulating factors therefore it coagulates much more rapidly, where it must be analyzed 2-8 hours after it has been centrifuged.