I did notice that most of the 767-300ER converted to cargo (P2F) are mainly CF6 powered.
Is there a particular reason why the CF6 is prefered instead of the PW ?
As Stich notes, if the operator has GEs, then they're likely to prefer GEs on additional conversions, all else being equal.
Problem is, all else isn't often equal, so some fleets also have PWs, because they might happen to be on the better of the airframes that are, at that moment, available for conversion.
The onesie-twosie feedstock fleet of 767-300BCFs and BDSFs that AAWW assembled for the Amazon assignment includes two maintenance spares, which are all-white, N1511A and N1619A. Both have Pratts.
These are Atlas-owned, not Titan/Andromeda frames leased to Amazon (despite the prime-number tail number with an A at the end). But they're used a lot on Amazon. In Peak, because Atlas owns them, they're on Peak duty, one for FedEx (1619A) and one for UPS (1511A).
Over at ABX Air, they have 3 767-300BDSFs that run Pratts: N219CY, N220CY, and N226CY, retired SAS ships that run PW4060s. The CEO was asked recently about engine choice, and his comment was that they have 3 at ABX with Pratts, that the GE and the Pratt are both excellent engines on this airplane, and that the Pratts do the missions just fine; it's just an added maintenance complexity to have two engine manufacturers. That said, DL does a lot of ATSG's engine work, and DL of course runs both GEs and Pratts.
One other point: the Pratts for the 767-300ER all have digital engine control (i.e. all PW4060s and the like). The GE's for that airframe come in two flavors, the CF6 with an F at the end and the CF6 without. The F stands for FADEC, which is GE's nomenclature for full-authority digital engine control. Throttle-by-wire if you will. Most (if not all) of the Titan/Andromeda leases to Amazon are FADEC. All of the ATSG (CAM) leases to Amazon run GEs, but all are non-FADEC. Part of the reason for that is that a lot came from AA, ANZ and QANTAS, which used the non-FADEC version.