Because Mario and Pokemon have been with the same developer for some 20 years now. Crash was on the top of the world with Naughty Dog. After them, it kept changing developer constantly, giving no consistence. In fact, the first Crash post-ND still sold pretty well - but consumers pretty much gave up after they saw it wasn't on par with the ND games. Spyro suffered from the exact same problem.
In Sonic's case, it was both Sega's colossal screw up with the Saturn, and their inability to transition Sonic into the 3rd dimension - not to mention an unnecessary focus on story.
Digimon had its biggest chance at its peak, with the first season, Digimon Adventure. But I think that Digimon World was not what people were expecting. If they released something more like Digimon World 3 or Digimon World Dawn/Dusk, perhaps things would be different? We'll never know
