Tarkin is a clone of Snoke. What better person to control the best fleets and the Death Star than a clone of yourself?
They have very similar physical options (face slopes right down to the fitting), and Grand Moff Tarkin’s grand look in Rogue One (whether you appreciated it or not) might be Disney coaxing the concept.
I got here up with this theory about 10 minutes ago, so bear with me if it is not very fleshed out but. I will replace this as new help for this idea involves me.
Edit: In fact, I’ll accept any criticism or proof from the established universe.
Edit 2: I remembered that in A brand new Hope, Darth Vader is commanded by Tarkin to NOT kill Admiral Motti. Perhaps Darth Vader knew that Tarkin was extra powerful than him. Perhaps Tarkin even used the power to “persuade” Darth Vader to not kill.
Somebody with an analogous concept has some extra convincing proof here https://www.reddit.com/r/starwarsspeculation/comments/3yrh6n/supreme_leader_snoke_is_grand_moff_tarkin/
Edit three: Here is a quote from A new Hope- Princess Leia Organa: Governor Tarkin. Men’s Cotton Luke Skywalker Short Sleeve Tee Shirt I should have expected to seek out you holding Vader’s leash. I recognized your foul stench when I used to be introduced onboard. Wilhuff Tarkin: Charming to the final. You don’t know the way hard I found it, signing the order to terminate your life.