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From an excellent Wikipedia Mercury Article

Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), as shown in the diagram below. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun. This occurs about every seven years on average.

enter image description here

I appreciate that the investigation of the formation and history of the solar system is still very much an open topic, so there may not be a simple answer as to why Mercury has such a highly inclined orbit.

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I may be wrong, but i think this may be related to orbital resonance effects. Due to orbital resonace, orbits are often destabilised and in some case stabilised as well. This inclination may have been produced over a long time, producing a more stable orbit(from the viewpoint of orbital resonance). For more info. See this https://en.wikipedia.org/wiki/Orbital_resonance

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  • $\begingroup$ Thanks very much. I must say, Wikipedia is particularly strong on astronomy. $\endgroup$ – user108787 Jul 12 '16 at 16:58

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