> If you have a diode, then the transistor is only a small step away.

It is not. We've had semiconductor diodes since 1874, but it took many decades to develop the solid state physics to understand how they worked and how to extend them. Crucially, you need some understanding of quantum theory (energy levels, Fermi distributions, etc), which was not developed until the 20s and 30s.

Even after they had the physics down, Shockley still spent over a decade unsuccessfully trying to get a FET to work (due to trapped charges which were not understood until the 50s). This is partially why the experimentalists, Bardeen and Brattain, are quoted alongside Shockley as the inventors of the transistor, even though Shockley had come up with a lot of the theory years before.