Add-ons. Not everything works with Prepar3d so people who have invested huge amounts of money in FSX add-ons that are not compatible with Prepar3d would be very hesitant to make the switch.
Also, the rumors of much improved performance might just be that. Rumors. At the core it's still FSX and many people are in fact reporting worse performance with Prepar3d if you look around popular sim forums like AVSIM. The experience doesn't seem to be consistently better for everyone, although at least the OOM/VAS issues seem to be all but gone.
It's a mixed bag. There's definitely quite a bit of research required before taking the Prepar3d plunge to check if any of your "must have add-ons" actually work with it.