There are 2 ways to do PNP. One is get an LMO, get a temporary work permit, go to Canada, then apply for PR through PNP. The other is get AEO, use AEO to apply for PR through PNP before coming to Canada.
I think more employers prefer the first method because they don't want to waste their time with somebody who might not end up working in their company. If they get them as temporary workers first, they can make sure they want to keep them before they help them apply for PR. Since the TFW can't change employment while the PR application is in process, the employer also makes sure he keeps them around for at least that long.