Because the person who wrote the tier list is biased? Clearly they suck at writing tier list as the entire point of a tier list is to either break things apart where there is an obvious divide or to spread heroes out using the tiers as sort of ranking.
Imperius is very strong. Especially late game he can deal thousands of AoE damage to clustered heroes very quickly. However he also has major weaknesses such as poor escape, iffy self-healing, limited range options and no support abilities.
On the other hand a Bruiser like Thrall has generally higher DPS during the early game, the ability to poke enemies, access to similar levels of disruption, fantastic self heal capabilities and is hyper mobile. Thrall can also give all allies periodic 33% shields thanks to Earthquake upgrade. Let us not forget his fantastic single target burst DPS. On the other hand Thrall does not have access to stun outside an ultimate and has nowhere near the AoE DPS at level 20.
On the other hand a Bruiser like Ragnaross will lose to both of them 1v1. However Ragnaross has far superior wave clear and macro than they do. His AoE damage is also very good and unlike Imperius it does not require level 20 to come online.
Most bruisers have strengths and weaknesses. There is no real tier list as a result since one bruiser will lose to another bruiser depending on the rest of their team comp and what map it is. One could make a Brawl tier list for them, but that would be dominated by Thrall as the only S tier due to how well he works in that sort of play.