I'm sure the availability of VHF channels in some areas plays an issue. Obviously they have received spectrum from the feds to build out some sites using 168-172 MHz frequencies...most of those are located right near NPS sites, not coincidentally. So far no VHF sites have been licensed in Pike County under either the commonwealth or First Energy, so perhaps they've already obtained more of that fed spectrum due to proximity to the DWG National Recreation Area, if the pattern continues.
I suspect that the other reason for some of these 800 MHz sites/cells has to do with existing county systems. Note that Chester, Montgomery, and York counties all have 700/800 P25 systems, and they all have 800 MHz sites/cells for the commonwealth's P25 system now.