1. When they blow, they need to be replced BUT they are so small (that an advantage) it can be difficult to find the offending transistor.
2. Manufacturing techniques are complex.
3. Manufacturing techniques require clean rooms.
4. They can be put into the circuit board the wrong way and therefore not work.
5. Removing them from a circuit board involves unsoldering whereas valves were plugged.
6. If they are on an integrated circuit and blow, the ENTIRE integrated circuit must be replaced.
Ummmmm...