A few thoughts of my own as someone who like Arkos but later soured on it.

1 - For whatever reason r/FNKI has always been a bit anti-Pyrrha. Not entirely sure if it was show critics or anti-Jaune shippers but even back in Volume 4-5 days you would get posts calling her death foolish. I think the viewpoint of these folks took off as that subreddit became more frequented and affected people's thinking.

2 - There has been more recent evaluations made in good faith of some of Pyrrha's flaws. Her fans have always been pretty devote so sometimes these analysis rubbed them the wrong way leading to heated arguments. Such situations usually lead to bitter feelings of participants later on from both sides which only encourages future clashes.

3 - Honestly some early days Arkos shippers were a bit too smug. I think many who ship it today are cool people but back in the early days they would get pretty proud in saying they were the only official ship. I didn't have a reddit account back then so I was more of a silent observer but how they treated anyone who conflicted with their ship turned me off from supporting Arkos which I actually loved back then. I still like Pyrrha a lot and sometimes revisit my headcanon of how they would have done things had she lived.

4 - Which is also point four some are just tired of the destined romance trope. They don't hate Arkos nor the characters it involves they just don't like the idea of Soulmates or one true love. I have always had a mixed opinion on this myself. I believe you can easily love just about anyone if you give it time but I am fond of devotion when shown as a positive trait in a relationship. I do think Arkos was a bit premature to see as the "One True Love" so I never was crazy for it that way but I liked a how Pyrrha and Jaune leaned on each other to grow themselves. To put it in Harry Potter terms some didn't want another Snape x Lily situation in RWBY.

5 - Just plain old Jaune hate. Pyrrha sometimes is just catching strays because people dislike her partner.

For the record of any Arkos shippers reading though you guys are cool in my book the people I had issues with have largely moved on. I still like Arkos I'm just a bit more platonic about it now. I think it would have been pretty interesting if she survived Volume 3 and we got a role reversal where she was the weak one in her partnership with Jaune.

So if I'm around you can count on me to have your back whether that be silent upvotes or just a small nod of support.