This does seem to be the caseRegional amusement parks have never really been known to have dark rides and it's also not what their guests come to those parks for. You don't see SeaWorld building dark rides either and that's because they've sort of become the closest thing Orlando has to a regional coaster park. SW has a bunch of APs and they come to the parks for the coasters, exhibits, festivals, etc. Just like regional parks, this is a lower spending group of people who are just looking for a fun time out and a lot of that includes riding coasters which is why that's what SW has focused on lately.
Disney and Universal build variations of dark rides all the time though. The reason for this is because the demographics that go to regional parks and what they expect there is completely different from what people expect at the top tier parks. A well done dark ride is what sets Universal and/or Disney apart from other parks. Rides like The Amazing Adventures of Spider-Man, Forbidden Journey, Rise of the Resistance, etc all are experiences you can only find at a park that has the resources at their disposal to pour into a dark ride. Even a bad dark ride like Superstar Limo rarely gets made outside of a larger park.
That doesn't mean that dark rides are dying, it means regional parks are allocating their yearly budgets to projects that their guests will enjoy rather than going all out and spending multiple years budgets on a dark ride that guests may or may not respond to.
But I have to think of parks like Blackpool Pleasure Beach and many of the Ghost Trains that are staples of Amusement Parks
It seems Sally has made the dark ride business a lot more accessible to regional parks...maybe we're just in a lull