Ride photos started out on thrill rides - Splash Mountain was first. Easier to sell them for thrill rides, and the pictures pop because of the thrill of the attraction.
In some cases, what you're talking about is technically difficult to accomplish. For example, there's nowhere to put a static camera around Dumbo that would make for a good photo, since the vehicles elevation is human-controlled. Putting it on the vehicle itself would be error-prone and expensive. (And people would probably complain about having a camera in their face the entire ride.)
You could theoretically put one somewhere on Peter Pan, but you'd probably need a flash at some point, and figure out how to do so without ruining the ride. Same with Ariel.
The biggest obstacle with all of that, at the moment, is that there's nowhere to put the PhotoPass monitors for people to get their photos after the ride. The places that have them now are mostly where they used to physically sell the photographs, or where the monitor areas were specifically designed into the attraction. Mermaid might have the most physical space for that, but it's not something that was considered when they built the attraction.
Once DLR gets MagicBand, that opens up more opportunities to add photos in rides here and there, since it would allow for photos to automatically be tagged to people's accounts, rendering those monitors unnecessary. (WDW has done this in recent years.) If that happens, they'd probably start with more of the thrill rides anyway - Pirates, Indy, Haunted Mansion. But that would at least make it possible.