You can actually run GPON and XGSPON simultaneously over the same PON segment as they use different wavelengths of light, so there is no reason to rip out all the GPON nodes at the same time. This allows deferring the truck roll and ONT costs until the customer upgrades to higher speeds.

With DOCSIS there is much more pressure to upgrade all CPE as any given chunk of RF spectrum can only run one version of DOCSIS. One 6MHz channel of RF spectrum on coax has a puny amount of bandwidth compared to a single lambda on fibre.