... the majority only let him serve one year. The rest cut him off at two. I'm sure the practices outside the US vary widely.
Two of my jurisdictions have one-year terms, the other has two one-year terms. I've read the law book for 2 of my 3 jurisdictions so far and there is no mention of term limits. It is local tradition not law.
In Illinois with a 2 year tradition it occasionally happens that a MWGM is so unpopular during his first year he gets voted out instead of serving the traditional 2 years. This has happened within living memory.
On the one hand I figure the traditional terms are because brothers line up to be in the grand line. On the other hand it appears to cost far more than is reimbursed so a MWGM has to be of independent means. Local lodge may be able to afford admitting brothers without regard to worldly wealth, but grand lodge can not afford that.