I always felt Backlund could drop the strap at anytime, to almost any challenger he was wrestling. All-American Boy Wonder vs. Super Monster Heel/Evil Wrestling Genius always left an element of "what could happen" on the ring apron, and Bob leading or dominating matches against his opponents never dispelled that vibe.
Hogan, on the other hand, I never felt could lose to anyone he was thrown in with. You had to learn to enjoy his matches for what they were and what was going to happen pretty quickly to make the match work in your head.
The better a heel challenger did against Hogan, even in defeat, was something that could elevate his status moving forward after the Hogan series. The Savage series in MSG is a good example.
No matter how well a heel challenger did against Bob, I don't think it ever helped them get over after the series was completed. Look at all of Greg Valentine's series' against Bob, where the title even got held up once in 1981. I don't think those matches did anything to elevate Greg after they were done in the eyes of the fans as he was just another failed challenger.
A lot of that is the ladder-style of booking during Bob's reign (climb to the top and challenge, lose, then climb back down and lose to Pedro, Putski, et al before leaving). The design was for the heel to be at his peak when he challenged Bob. Same thinking wasn't in effect so much with Hogan because nobody really expected a challenger to beat him. After the Hogan challenge, guys could still be strong against the level below Hulk.
Given that they were keeping everyone for long periods of time in the Hogan era, it was the only thing they could do. The Bob-Style ladder wasn't really feasible.