Hello Everyone.
I would like to share my thoughts about it, and then receive your feedback.
For me, the answer is simple. We use semiconductor because we wish a material that you can control easily the conductivity. And for this purpose, semiconductors are the best one. It is not so easy to control the conductivity of isolator and conductors.
The transistor works as a switch, that is close (low resistance) or open (high resistance). It is easy to do this with semiconductors. You create the channel with a gate-bulk (or gate-source, …) voltage, and then, you close the switch.
These are my thoughts.
Best regards,