It's about spending money on what will bring you a return and what is marketable, because ultimately that is what allows you to make your return.
Merlin have seemingly concluded that the costs associated with the purchase, marketing, staffing and maintenance of flat rides is not workable for their parks at the moment, hence we have seen few additions and a good number of removals. Six Flags & Cedar Fair went through the exact same thing but are now actively seeking to add flat rides to their line ups - so perhaps the economics are changing and also because a good chunk of their parks had become great for coasters and not much else.
I think a BIG flat ride doesn't suffer from the same problems but planning is obviously a consideration there for Alton. Plus, if you do have a hit on your hands and your new flat ride becomes a headline grabbing attraction (for the right reasons) - chances are it's not going to shift many more than 400 people per hour with a reasonable cycle time.
Yum! own KFC, Pizza Hut, Taco Bell etc. They have a certain amount of money that they spend on new outlets. If they're going to spend £1m on a new restaurant in a location, they will open the brand that allows them to generate a return on that invested capital within their predefined period - which is why, in the UK - you see less Taco Bells than you see Pizza Huts and KFCs. That's not to say Taco Bells don't make money - they just make less £1s per £1 spent so that chain remains comparatively small.