There is no real reason. Base 10 as the default system probably comes from counting with fingers, but there's no reason we couldn't do math in base-12 if we counted with joints instead.

Ancient Babylonians counted with base-60. We still measure time in increments of 12 and 60 because of that, but also because both 12 and 60 are convenient bases to work with since they're easily divisible by 2, 3, 4 and 6.