I would rather not have advertising services emailing while I explore abandoned facilities and secluded caves. That would ruin the atmosphere of being lone scavenger.
On the other hand, it makes perfect sense to have comm device in black sea, where I'm hired mercenary working with large team of people.
And black sea, as already pointed, is open space. Rest of underrail is narrow corridors drilled in thick stone. This is reasonable explanation for why comm devices are not used that much there.
