I think my thought process of it is that coming just short of the SB leaves a "we've got more work to do, but we're on the cusp of making it happen. Time to grind." while getting embarrassed in the SB has a finality feeling to it. Obviously this can be overcome as well, but some things linger longer. I think of the Falcons and the Panthers. Both embarrassed in the SB, both have been up and down (though mostly down) since then. That being said, the Broncos got embarrassed in the SB and then got right back within a couple years, but that was with one of the greatest professionals to play the game at QB, and with a new coaching staff.
I feel like it's easier to go into the next season after a close loss, rather than a blow out.