There are three parts that determine how fast a tank can go: the engine, the transmission, and the suspension.

The engine is the most obvious part. Tanks are heavy, and in order to make them go fast, you need a lot of power. Automotive technology at the time was not particularly advanced, so if you wanted to have an engine small enough to fit into a tank, you had to compromise on horsepower. The French FCM 2C tank opted for the biggest engine possible, weight be damned, and the result was a huge bulging engine compartment that prevented the turret from turning backwards (requiring another turret on the tail of the tank), and even that tank achieved a measly 7.1 hp/ton.

Then there's the transmission. The power the engine generates has to get to the tracks somehow. The more power it passes on, the more robust and complex the transmission has to be. WWI tanks were hardly pinnacles of reliability to begin with, and pushing more power through them would degrade that even more.

Finally, the suspension. The suspension of WWI tanks was unsprung, meaning that the impacts from driving over uneven terrain weren't dissipated at all. This had two disadvantages: one was that the crew was shaken about like flakes in a snowglobe, which tired them out quickly, the other was that if you went too fast, the suspension would smash itself to pieces. Figuring out a proper sprung suspension is harder than it looks, and nobody really figured out a very good way to do it until long after the end of WWI.