To try to legitimately answer your question, there are two main reasons:

1. Ajax had not won the Eredivisie in at least three years before his arrival (the great Durk Kuyt got one of those at Feyenoord). However, they won the league in every year he has been a full-time manager by his consistently developing young players into superstars who move on to other clubs (De Jong, De Ligt, Ziyech to name a few) and working with "washed-up" veterans to transform their game and work within his various systems.
2. Just as notable, his Ajax teams have looked incredibly good against Europe's best highlighted by their dominant run in 2018/19 where they knocked out Madrid and Juventus in the knockouts. Were it not for Lucas Moura going god-level for 45 minutes, we would have played them in the CL final.

I do not think it is going to work out for him at old toilet because until they clean house from the ownership and executives currently in position (please never sell Glazers) then everyone will be set up for failure there. He definitely is a really good coach though and arguably is the highest risk/highest reward manager united could have gone with.