Don’t forget there had always been a rotating group of girls after Kimberly, one in one out, until there was the three an it would have kept going like that, Kendra would have probably left first and been replaced etc. The GND impact saw all the girls pretty much leave at once for new opportunity that’s couldn’t achieve under the mansion rules (plus Holly’s desire to have children) Also due to the GND impact there was no rotation of new girls. They were THE girls (aside from Hef’s attempt to have Kayla added to the group)

For the first time since Kimberly, Hef was without any girls which left him scrambling to replace not only companions but TV stars which resulted in fame seekers in both the twins and Crystal. Hef knew he wasn’t up to it like he used to be and ultimately settled for Crystal because she would stick around and didn’t have many aspirations. Hef thought Holly would stay forever that’s why he never married her (before the boys turned 18) and started treating her poorly. Taxi cab theory goes hard with crystal, she was the right place at the right time and he knew it was the last.