Prot Paladin had an overwhelming dominance over any other tank back in S1. As a result, they got nerfed. The only other tank that has surpassed Prot Pal Dominance last tier in M+ history was DF S3/S4 VDH which was slightly stronger due to double sigils.
Usually when a spec its so dominant, they receive a substantial nerfs that last for at least half a tier or sometimes even more, as a healer main recent examples of this are: DF S2 Hpal, DF S3 MW, DF S4 Rdruid, you get the idea. Of course not saying this is good or ideal, but its usually what happens every time a spec is this strong for such extended periods of time (with the exception of current Disc priest/VDH, so far atleast…)