Because HOF voters like doing silly things like this:
He finished with 493 career homeruns, 27th all time.
Of the 26 ahead, 9 are either active or ineligible.
Of the remaining 17, 16 are in the HOF, Mark McGwire is the only one not elected.
The good (?) news for McGriff is he's ahead of the curve where guys ahead of him are eligible.