When tubes are driven to overload they clip smoothly and compress the sound giving us the distortion we all know and love. Transistors clip very harshly with a crackling distortion and can deliver it's power supply voltage directly to your speakers thereby frying the voice coils. Preamp tubes can be replaced by the user and many newer models have user adjustable bias, allowing them to replace the power tubes on their own. Transistor amps are not user serviceable. Quality tube amps hold there value.
Vintage units demand a high price that is well deserved.
Transistor amps depreciate quickly and are difficult to sell. Tubes are nice to look at and have a lot of character. Transistors are as ugly as they are boring.