I'm not especially knowledgeable on Hariyama, but my guess is it suffers from a similar case as Hitmonchan - competition. Hariyama is a threatening Guts sweeper, but at the same time, Medicham hits plenty hard already even without a boost almost as if it actually had a Guts boost, Emboar gets that excellent alternative STAB in Fire to threaten more, and Gallade gets Swords Dance to be a harder hitter and/or have Drain Punch to keep itself alive. Its good bulk is also mitigated by the status orb it is using due to the recoil and its the slowest Fighting-type in the tier. The defensive set's main problem is similar - competition from Poliwrath, mainly due to the additional Water-typing giving it the same resistances as Thick Fat Hariyama and more and Circle Throw being a two-in-one deal.
Hariyama isn't bad though. It is extremely hard to switch into because it can be a massive threat pretty quickly because it doesn't need to set up like Gallade due to Guts never going away and it is much bulkier than the likes of Medicham, meaning it can take a hit if it needs to. Whirlwind also comes in handy against Hail Pokemon, especially Walrein, and not being weak to Thunderbolt and Grass moves helps against a myraid of special attackers, notably Magmortar and also random stuff like Manectric and some Sceptile when compared to Poliwrath. The advantages are what make it worth using, but the disadvantages make it come across as somewhat inferior to other Fighting-types, although it is still threatening.
Perhaps the biggest reason gets a bad reputation due to inexperienced people using Fake Out mindlessly on the Guts set, which provides safe switch-ins a lot for the opponent. Fake Out is useful, but it isn't a get-out-of-jail free card. Granted, this is much more common in ladder play than in a tournament environment.
I hope that makes sense :/