When an aircraft lands, the nose wheel remains elevated, and the main wheels touch down first. This results in a considerable impact on the runway as all of the weight of the aircraft hits the ground. In order to minimise the impact, and prevent damage to the runway, larger aircraft tend to have multiple bogeys on the main wheels to distribute the weight over a larger surface area.
Due to the sheer size, and unique mission of the An-225, it features a number of wheels in order to maximise the weight distribution on the runway surface, allowing it to carry a greater weight, and operate from smaller airports with weaker runways.
The nosewheel does not suffer from this problem, as the landing impact is much softer due to the main wheels taking most of the weight.
Interesting aside: The A320 can be purchased in both a single-pair and dual-pair main wheel configuration. So far, Indian Airlines (now part of Air India) are the only airline to operate the dual-pair main-wheeled A320, as the standard of runways is not as good in India, and therefore they can serve airports they otherwise wouldnt be able to do.