Trump has been successful in recruiting politicians like Pence, whose career had plateaued and was suffering from low approval ratings (see also, Nikki Haley) or were already termed out of office with no obvious place to go (Sonny Perdue) and also the already super wealthy (Tillerson and Mnuchin). 

But there is a second strata of positions below the level of Cabinet Secretary and related positions. At that level, the level of the real subject matter experts, he has struggled to find people for roles like solicitor general or deputy cabinet members specializing in a policy area. Why? Because that class of people still have something to lose. And they believe that they will end up losing by joining his administration. Those who ask themselves, what kind of job will I be able to get after I serve Trump are implicitly answering that too many potential clients won’t want to hire someone tainted by association with Trump.