Need of semiconductor instead of conductors for changing conductivity
&quot;If my grandmother had wheels, she'd be a wagon&quot;.
The semiconductors are used because of their particular properties. This is usually silicon, but doesn't have to be; see germanium diodes, gallium arsenide, cadmium telluride, silicon carbide and so on.

When we can change conductivity of metals(by some means)

It's not even clear what tense this is in; do you mean we can currently change the conductivity of metals? I don't think so. Or if something in the future is developed?
What do you call a metal whose conductivity can be changed? A semiconductor.