Why blood and frost are not viable in pvp?

Im not questioning this, im just a new player that plays blood and im still leveling, and I want to learn something.

Why is it that unholy is so much better than other specs in pvp? Is it because of the ghoul and his stun? is it because of gargoyle burst? I dont think its a case of dmg as in pve Frost and unholy are both competetive in PVE. Blood has some nice cooldowns when dueling stuff that give him increased healing so he is not that easy to kill, which i would think would be good in arena. Maybe blood is requied to be in meele to deal dmg and thats why he is not good?

Blood and Frost are still "viable", they're just not as good in an arena setting. In a BG, it's debateable, especially if you are queued with a team that works with you.

Unholy has the best "pvp sustain" because once the diseases are up, it's pretty easy to maintain some pressure at range. Unholy also has the best multi-target pressure. The Ghoul stun is useful, so is Desecration and Anti-Magic Zone. Unholy also has the ability to make their diseases undispellable. Unholy also has the perk of being able to heal just as much as Blood off a Death Strike once diseases are up.

Frost doesn't really have much in the way of tools that an Unholy spec can't reach. You can freeze people, but that's broken on any amount of damage. And then there's Unbreakable Armor, which is nice as a damage CD, but useless as a defensive against casters.

Blood has nice self-healing, but that isn't really necessary if you are playing with a healer. Hysteria isn't considered to be very good in arena because it causes the target to take additional damage, but it can have its uses in a BG. Both Blood and Frost suffer from not being able to stick to a target as well as Unholy, and their diseases are easily dispellable. If you're up against a Paladin, RIP your damage. Even worse against a Priest, since they can "auto dispel" your diseases.

Thanks, that explains a lot.

Frost is very viable. It's sort of glass-cannon type of spec.

I have observed a lot of low skilled DKs having more success in arena after going Frost than as Unholy. Hungering Cold (10 sec cc every 1 min) combined with better burst allows you to kill opponents more reliably (at cost of survivablity).

Unholy has better overall survability, control in a matter of both CC and slows, and wins most classes in 1v1 scenarios.
It also has good constant DMG pressure and awesome DMG with Garg and DRW.
This is why its best for soloque rbg, world pvp, and arena.

Frost has good 10sec CC with 1min CD with a perfect setup using DG, and an awesome burst.
Downside of Frost is less survability than Unholy and casters can kite it.
Duo to CC and burst, Frost is good in 2v2 when playing with a Healer, HPala for example.

Blood has good "survability" but u can kite it easly, and if u kite it, DK cant heal.
Spec, Blood, is mainly good VS Ret Paladins, the only class Unholy DK cant beat 1v1.
Other than that, i really dont see why would anyone play Blood cos Unholy is simply better.

When it comes to Blackrock I would say that Frost and Unholy are on a par in PvP. In Patch 3.3.5b Frost was the strongest 2v2 spec, FDK+Hpala was even outperforming Warr+Hpala.
On Icecrown Unholy is slightly better than Frost due to Smourne.

In terms of 2vs2 Frost is better against melee setups while Unholy is better against caster setups.
In 1v1 Unholy is always the better choice.