Why does the midi system have the widest range of frequencies?

The MIDI (Musical Instrument Digital Interface) system has nothing to do with the range of frequencies.

MIDI is a method of communicating between various musical controllers, such as keyboards, and generators, such as synthesizers. The synthesizer is what actually generates the frequency. The MIDI system is simply the messaging subsystem. Think of it this way - one parallel to MIDI is a player piano roll.