I don't know his reasons but I'll throw my thoughts in on as to why someone might. From a Prot warrior perspective we are designed horribly in this expansion. Currently we are a viable tank but as the content scales we will fall further and further behind.
1. Rage from Damage Taken - This mechanic is awful, it was awful in Vanilla when warriors had to strip gear on purpose to take more damage for rage and it has been shitty every time they try and bring it back. No other tank is forced to deal with this bullshit as they all have control over their own resources. It actually punishes you as your mitigation increases.
2. Ignore Pain - Ignore pain is a very flawed design and is/will suffer from the same issue shield barrier had, it scales very fucking poorly and even more so after the 33% nerf. While morons were throwing shit fits that warriors were jesus on 5 man normals due to Ignore Pain they forgot to look at the fact that it's scaling is terrible and already in Heroic EN+ , and higher end Mythics you are better off gearing for haste to lower your GCD to maximize shield block over even thinking about ignore pain. This issue is compounded by the fact that almost every aspect of our shitty artifact is tied to this bag of shit ability. What is even worse was how this ability would show up on healing meters leading dipshits to think warriors had actual healing. We are the only tanks with no viable self healing mechanic.
3. Legendaries - The warrior Prot legendaries are pretty meh in total, we have no game changing legendary item like some of the other classes have such as Guardian Druids with their 2 swipe legendaries that have amazing senergy with one another and increase their mitigation a significant amount. Our best legendary simply gives us a small bonus to devastate and shield slam.
4. DPS/Utility - In this area we are the weakest of all the tanks, we have the lowest single target and AoE DPS of all the tanks as well as bringing very little utility. We take hits to the face and that's about it, and Bliz won't even let us be that best at that. When you look at the current state of the game which is mostly Mythic/Mythic+ content single target a Warrior will hit 150k vs a Druid 200k that's a 25% difference which is made even worse when you see their AoE damage compared to ours. We don't bring the utility such as Battle Rez, or group movement speed like druids do. When so much of the game is based around the piss poor diablo3 greater rift design where speed is everything that utility is so important.
So while we are in a viable position now as the other tanks start filling out all their traits in the Artifacts, as the gear scales higher we're going to fall further and further behind. Already the geared paladins are starting to surpass warriors and druids are straight up top in every department even though they are meant to be jack of all trades masters of none instead of this fucking best at everything in all areas bullshit we have now. I still think there needs to be some balance passes to DK's, DH's, and Brewmasters. Although the more I look at DH's and them being the new tanks on the block I think it's a player issue as so many opt for extra self healing over better mitigation and higher uptime of Demon Spikes but that's another story.
Our rotation is very rigid and boring, we are pretty much locked into Ultimatum because of rage starvation at all times since the nerfs, and because Rage from damage taken is a shit mechanic, and lets not even mention the extra rage joke of talents in our artifact. I enjoy the class for what it is and have played Warrior through up's and down's since Vanilla but it's hard to want to in the current state.
Now on the other side for DPS, warriors suffer from the same utility issues but I think they are in a better spot that people like to admit, yes they are middle of the pack and subject to some RNG but so are specs like outlaw. They recently buffed fury and lowered the extra damage taken which was a good choice as their is no real reason they should be subject to that horseshit design choice. Right now warriors are abysmal in PvP which if that's something of interest to you then I would reroll off Warrior in a heartbeat.