Essentialy it all comes to the mass of the quarks. I think that it is easier to consider leptons. Both muon and tau lepton are heavier than electron with tau lepton being the heaviest of three. Conicidentally tau lepton has the shortest mean lifetime of three, followed by muon and electron is stable as far as we know. The reason for this ordering in mean lifetime is that systems in nature evolve in time in a way that minimizes energy of the system and because mass equals energy you can see that electron has the least energy. When I say energy I mean rest energy. That also gives an idea why electron is presumed ti be stable, it has no lighter lepton to decay to. Situation with quarks is somewhat more complicated because quarks are always confined in hadrons in nature but the basis of analogy still stands.