DPS is not the principle factor in PvP, it’s a combination of burst, survivability and utility (both defensive and aggressive).
For example ret is a mediocre pve spec at current (according to player estimations) yet is one of the best pvp specs at current. Why? Because it has a strong burst phase, exceptional utility and a valuable immunity effect.
Arms is mediocre in pve compared to the likes of MM, Boomkin, WW, OL. Yet it is the number 1 melee dps spec to run with a healer in 2s for example. Why? Because despite it lacking the damage and even burst of other decent specs, it has a truckload of utility, is very tough to kill, exceptionally hard to peel and is very good at outlasting opponents and playing slow until a kill opportunity can be created on the enemy healer (whilst creating pressure of its own upon them).
Healers too. Holy Paladin is performing okay in PvE for sure, but it ain’t no holy priest/disc priest in raids and it ain’t no resto shaman in M+, it’s an all rounder. Commonly regarded as the best healer in 2s and 3s though, why’s that?
It might lack some of the raw HPS other healer specs have over the encounter, but it is tough to kill and has a lifesaver defensive in DS which is absolute gold on a healer because healers are typically a kill target otherwise. It has two emergency cooldowns for allies to save them from being killed instantly if selectively (Blessing of Protection and Blessing of Sacrifice w/ Ultimate Sacrifice). It can make a target immune to snares and peel with Freedom. It has a 6 sec stun which is extremely good for setting up kills if you can land it without DR. Beacon of Faith in 2s and 3s means a majority if not everyone on the Paladin’s team is receiving healing whenever they heal, and in conjunction with Saved by the Light means everyone effectively gets an emergency shield when approaching death.
With Avenging Wrath doubly so with Venthyr) the HPala can also put out notable burst when required, albeit for a small window, again this helps to secure kills. This is of marginal use in raids and M+ (I’d be better of healing to keep people up) but in PvP the fact I can crit a guy for 14k instantly with one spell is significant. If I opt to invest more time in burst (and crits favour me) I can take that burst up to about 30k within a few seconds which is a massive boon to gaining a kill, which is what rated pvp (arena) is all about, as the kill massively changes the outcome of the game.
Yes, good DPS is often an asset for sure in PvP I won’t try to deny it, but when you look at the best classes according to the rated meta, you will see their burst is largely but one of many things they offer, and were their burst on it’s own, it would be regarded as fairly meh. Take mages, Arcane can put out astronomical burst if they need to, but they aren’t mobile whilst doing it (utility) whereas Fire is. Fire also has cauterize and Arcane doesnt, and Fire also has Ignite to keep people from destealthing on them and Arcane doesn’t. Fire is picked over Arcane every single time in PvP.
It’s the same with Sub rogues. Sub rogues can burst yes, but their sustained damage is woeful compared to Outlaw. But their main calling cards are they can reapply stuns across the enemy team frequently and they can isolate a target for kill with duel preventing a healer clutch to secure it. This is invaluable. If they couldn’t use cheap shot repeatedly nor duel, i’m not sure you’d see nearly as many sub rogues at all because their burst can be countered but it;'s their utility to makes them continually dangerous even if you mitigate their burst. Their burst target gets BoPed? NP, they can just step to new target and being stunlocking them and call for a swap. No other rogue can do that without spending their combo points (killing their own damage in the process).