My wild-ass guess here is that some filtering effect is in play. Secret service people that can perform at that level, with a clean enough background, can absolutely move on to a better (and less lethal) job if they want^1^. This is a position where "will you take a bullet for this president?" is absolutely front-and-center for the interview process. Also, it doesn't take a brainiac to see that this President has a tendency to attract violence, which is not ideal for anyone with that security detail. So, trump got the best security he could possibly get which, due to his repugnant personality, are probably not the best in the business.

^1^ "Our new company head of security is former Secret Service, can you believe it?"