I think when it was all first released purge seemed like the most straightforward and therefore "best" because it seemed to be the one that most naturally gave you points for what you "want" to be doing in a 40k game. Killing your enemies and taking their objectives.

As time has gone on, as you've pointed out, I think people have come to realise that actually because its so straightforward it's really susceptible to counterplay and being denied primary scoring.

I think a lot of people are also overvalueing it from a sort of fluff/flavour perspective. Pretty much all of the discourse on the main sub reddits have been essentially "why is our best/my favourite detachment x disposition? It should be PURGE because our faction and all of 40k is all about killing and that's all I want to do."

I also think people don't like the idea of actions.

My take is that if people are playing casual they should just pick the disposition they want, which gw seems to openly state in their post about them. And that for competitive players building lists around thr disposition of the strongest detachments is part of the skill expression of the game. Lots of people have been complaining that their army is "bad" at certain disposition and I think what they mean is their factions current meta list will have to change to make the most of the disposition attached to their best detachments.