hell, because he's always in the back seat while the other kages shine, even tobirama simply comes off as more powerful, Hiruzen seems like an old guy that's there for the ride, Hashirama is god of shinobi when talking about heavy hitters and such, this was said of him in part 2 where the scaling is outrageous when concerning power.
Hiruzen was called god of shinobi in part 1, when pretty much everyone on there is fodder, at least when comparing them to part 2, ppl even used the substitution jutsu all the time, naruto and sasuke were some of the strongest characters by the series end with just a one tailed cloak and cm, and no one even mentions hiruzen or his abilities AT ALL. Honestly I think at least one person would make a reference to his power, if he was god of shinobi, but no one does. Even Minato has demonstrated more than enough to effectively place himself in a position higher than hiruzen, until he shows more, he battles with tsunade for 5th strongest hokage.