Oh let me just stand behind minions. Boom, you are now immune to Tahm's Q. His Q also has quite the wind up time to it so it's actually easy to dodge. And Gap Closing is waaaayyyyy more important than poke when it comes to a tank.
A 20% slow isn't go t do much when if your opponent has any sort of mobility, and the only way you're going to get close is if you land 4 Q's in a row, or they decide to go close to farm/engage. I've played Tahm on PBE, and the only time I reliably devour'd someone was when they were already in my face (During laning phase)
During the teamfights, unless you built some AS, you're going to need to stick on a target for about 1 q and 2 aa's. An approximate 2 seconds given his AS. By that time, a carry might've been bursted down, a tank might've cc'd your ass, or the target got away from you.
What I am getting at is Shen's cc and team fight potential is much more reliable since he can get up close and taunt immediately, causing a huge sway in the battle immediately. He can taunt a tank, and boom the back line is now vulnerable. By the time Tahm can swallow a tank, let alone get close to one, the battle has already started, so what good is eating a tank if your carries are dead.
And if you decide to try to save your carries instead of being front line, you effectively put you and your carry out of the fight in exchange for his safety. Effectively making it a 3/4 v 5, the enemy now has the ability to take on your front line without the fear of a carry killing em.
TL;DR Shen's utility is more immediate in a fight than Tahm's and is much more game changing, since the first moments of a fight decide more than the latter part.