I find it ridiculous that the single most powerful being in existence aside from the daedric lords would choose to make his return to the real world revolve around the pewny island of Solstheim.
Because he was still trapped inside a realm of Oblivion and therefore, his power could not be fully manifested, anywhere on Nirn. Solstheim, besides being the island he once ruled over, with his center of power (his temple) and his cult, is also small. Which means he could control it's population much easier, by adapting the stones on the island, to some sort of transmitters/amplifiers, making his control over the population, permanent. And then, he would return.
Spoiler Until then, he would control them through some form of telepathic takeover, occurring whenever they went to sleep. Even the player will be taken over by Miraak, if he chooses to rent a room and leave the search for tomorrow. As I've seen, you find yourself working on Miraak's temple, woken up by Frea's pleas to her fellow Skaal, also working alongside with you. Apparently, because you're dragonborn, Miraak's control over you is loose and that's why you can hear her and break free.