Why are so many people against Arena in Swtor?

Because they come from WoW, and in WoW there was a very widespread - but very mistaken - perception that every single change to class mechanics and numbers was due solely to PvP.

"Stop nerfing my PvE because of Arena!" was a very, very common cry from people. They usually ignored the fact that most changes were due to PvE concerns as well.

The Warrior Shield Slam damage nerf was a good example. The PvE community whined and whined that their PvE was being nerfed because of PvP, even though Blizzard very clearly stated that they wanted to spread out Prot Warrior damage across their other abilities rather than having 80% of Prot Warrior damage in PvE come from one button.

It's funny, because in general PvP balance and fixes were practically ignored by Blizzard. Legendary quality PvE items in PvP, Arena maps that STILL had serious terrain bugs (Feared into the wall again, yay...) and other issues that just never got addressed.

So now we have these people in SWTOR, and they're just petrified of the idea of any sort of organized, competitive PvP because they still think that their PvE DPS/Healing/etc will get nerfed into the ground if anything happens in PvP.

Anyone who was successful at arena enjoyed it. There is a large population of people who want it. If there is no arena in STWOR people will move on to Guild Wars where they will have arena style pvp.

People who do not want arena don't need to participate. Arena is important for the longevity of a game because PvP is the only non static content in these MMOs. Implemented right, arena would be a great asset to the game.

Arena is not pvp.

I was succesfull and i didnt enjoyed it...
What sane person enjoys being cc'ed as warrior by resto druid in tbc arenas? Games that were lasting 15+ .... so much joy.
Not to mention that as warrior the only viable combo of all the classes out there, was a resto druid....And all those ppl that had warrior but their irl friend that played arena with wasnt resto druid so they couldnt top arenas....they had to be called scrubs and useless and bads....

About the 2nd thing..."People who do not want arena don't need to participate" Sure, but if some combo proved to be pretty much OP, dont u ever try to whine about nerf it...or buff it.
Cause the same ppl that dont play arena, they will be affected by the buff or nerf of their class in pve...
I personaly quit 3 class in pve (ret pala, dk, mage) cause blizz decide to change them for the sake of arena only...

Why have arena? We have Warzones, open PvP which hopefully some day will get fixed. Add more variety of Warzones later on but not Arenas, not needed.

That's super cute that you think War/Druid was the only comp worth playing as a Warrior. If it was the only one viable for you, then you were a bad player.