Teemo will work against Tryndamere until lvl 9 or so. And then after he lands a big item and some sustain you will never be able to do anything to him anymore as Teemo.
It's similar to say, Teemo vs Malphite. People think Teemo counters Malphite. Yes, he does. For the first bit of the game. And then after one big item and a decent defense item, Teemo will never be able to do anything to Malphite anymore. --- In the chronicles of my life there is a legend in which I change 'history' into 'herstory'.
Why should I play Rammus and wait until 6 to do anything, with that anything only being making him CS at tower which he can easily do, when I can play any other top, ward, and shut him down from level 1?
Also, Trynd can spin over a wall to foil the "follow him and taunt him" plan.
I just did rammus vs trynd, it was one of the most hilarious game I've ever had. He couldn't do any damage to me and just left top as soon as I got turret. Their team was full ad too which is why I decided to try it. Ended up doing most damage in the game too.