It's no idea to compare an award show, which was in a completely different format back then, people say Pac and Big have zero.
They only had like 2 chances to win and that was for singles. I don't know if they could defeat Mary and Meth or Coolio, it isn't like those were weak records.
And also competition is different.
Marvin was going against Stevie, Aretha, Temptations, Earth win and fire, Ray Charles and them, it's not like he lost to some off brand weak artist. And it was for best song, not even song but "Vocal performance" not album in the beginning.
Like always with these things people talk but can't specify exactly what they mean.
Yes he should've won 1973 for What's going on though, "Concert for Bangladesh"
But going of "classic albums", other albums that weren't even nominated were Stones Exile To Main Street, Bowies Ziggystar dust and Led Zeppelin 4 which are frequently mentioned among top 10 albums of all times.