Maybe Trump hired Dershowitz because he is a great lawyer
Dershowitz spent most of his career at Harvard Law School where in 1967, at the age of 28, he became the youngest full professor of law in its history.
Dershowitz sees himself as a "lawyer of last resort" - someone to turn to when the defendant has few other legal options - and takes those cases that are what he calls "the most challenging, the most difficult and precedent-setting cases".[22] He is currently advising Julian Assange's legal team.[23]
now he's advising Trump's legal team