Here's why:
* Planetary Alignment: For a transit to occur, a planet must pass directly between the Sun and Earth. This alignment is very rare and only happens with Venus and Mercury.
* Orbital Plane: Venus and Mercury have orbital planes that are slightly tilted relative to Earth's orbital plane. This means that they usually pass above or below the Sun from our viewpoint. However, a transit occurs when Venus or Mercury crosses Earth's orbital plane while it's also between the Sun and Earth.
While Mercury also transits the Sun, it's much smaller and harder to see without specialized equipment.
