my guess is its easier to jailbreak a appletv 4 compared to the 4k, there are more firmwares capable of it now vs then and people like kodi with less than legal addons.
i know a few people here in canada that did this for that reason and the main reason they went with AppleTV vs a fireTV, Mibox or some random chinese box was because Airplay, less ads and UI jank stuff like that.