I think it's because INFPs are the softest people persons of all and are the least judgmental when they meet new people. ENFPs are open, but we're not as out there. We lead with our Ne, not with Fi. INFJs have a strong protective shell.
Not sure about ENFJs. I think some of them could be part of the weirdo pack. And according to PersonalityPage, their natural partner is the INFP.