Because the job of stabilising a chaotic club mid season when things are going wrong is different from the job of setting up a club up for a successful season. Pereira proved he could do the former at Wolves but when he tried to do the latter it went horribly wrong. Not entirely his fault - the player losses hurt them a lot - but Wolves have improved significantly since he left, which says something.
For what it’s worth, I think he’s done enough that we have to keep him on next season and I still suspect that four managers in a season means we are too damaged goods to attract anyone significantly better. (Silva might be the exception because of his prior relationship with EM.)
Also Pereira’s track record is that he doesn’t stick around anywhere long. Maybe what happened at Wolves is why and he just goes stale for the players really quickly. Is there substance below the chirpy chappy easy fixes persona?