Raya is just better in every category that matters to Arteta who trusts him to carry out the game plan. And the eye test backs up the stats on this.
Mikel does not care about the errors either - he’s said as much publicly. What Arteta is basically telling his keepers is “I want you to take risks with the ball. If you screw up then it’s my fault, not yours.”
For the keeper it’s all mental when out on the field - to not be scared, to be willing to take the chance and execute what the manager is asking for even if every game you make an error and people wanted you dropped. Doesn’t matter to Arteta - all he will say is “get back up and keep playing like I want.”
What we saw from Ramsdale in the run in last season is that I don’t think he could handle that mentally. And Arteta got fed up with it. The loss to Brighton at home last season was the final straw when Ramsdale kept knocking it long so we couldn’t get a foothold in the game.
There’s a notable video of Pep in training basically saying the same thing generally to his players. https://x.com/KeyframeSports/status/1686669393504190464?s=20
Lol Ramsdale did this to himself last season.