The probably most simple explaination would be that for some roles you like to have someone that can be easier "shaped" into a certain role. Someone who is already successful may bring their own system of doing things. This is great if it is a good fit, but can produce frictions if it isn't.
The next thing is that if you apply to a mediocre position with overly amazing credentials, it can raise suspicions. Something must be wrong with you, maybe you got amazing credentials, but you are complicated to work with. Maybe you're looking for the mediocre job just because you think it will be a walk in the park, etc. There are legit reasons for this (e.g. "my partner moved to $TOWN for her career and I am looking for something to do here, and you seem like the best fit. I know I am technically overqualified, but I wanted to go back to coding for years now and this offers me a geeat chance to give it a go").
Of all the senior canidates we have rejected the most common issue was that they didn't offer a convincing explanation to why they chose that specific position. The worst one was talking about how it would be a relaxing position for them.
