My initial question was "What determines if a page can or can't be displayed in Reader View in Firefox?" but I found the answer on a Webmasters StackExchange question here.
As far as I understand, sites that don't automatically show the option to enable Reader View have a problem with their code/tag/html/something. The Firefox support page for Reader View (here) suggests something similar: "If a page is available in Reader View, the Reader View icon will appear in the address bar".
My question now is, if Reader View icon doesn't appear,
Is there a way to force enable Reader View on such pages?
Visited this discussion on Reddit but it doesn't answer my question.
I considered posting this question in Software Recommendations but I'm not sure an addon will provide a solution. Maybe this can be done with a script or a Firefox setting in about:config or something else entirely?