It is mostly ascribed to the covalent bonding of the catalytically active heteroatom to the carbon structure, unlike the physical bonding of Pt to carbon support (10.1016/j.rser.2015.12.330)
In my opinion it is due to the instability of Carbon based materials, as the Carbon corrosion is challenging problem in acidic media so very often the carbon materials are employed in acidic media until with modified structure e.g, CNTs, g-C etc.