Why can't tornadoes be stopped?

Tornadoes and the mechanisms behind them are large and powerful and they are not fully understood. Any means that might be devised to stop a tornado would require an enormous amount of materials and/or energy which would be impossible to assemble in the right place at the right time.

There have been proposals of nuking tornadoes in an attempt to stop them, but the explosion and fallout would cause much more damage and death than any tornado could and even then scientists don't know how a nuclear blast would affect a storm.