I suspect much of it has to do with budgets and expectations, too. For something like TRON, you can't get away with cheap f/x. By contrast, a film like Mad Max can be done with mostly practical stunts, and relatively limited post-production F/X. Sure, you can do it all with CGI, but you can still keep your budget low.
But if your budget is, like, +$100M, and you only make 30% of that back in your opening weekend domestically, you're screwed. No sequel for you. Why? Because numbers. After the first weekend, unless your film retains the bulk of its audience and your returns are around another 20-25% of the budget, your film is a financial failure. Most of the time, 2nd weekend returns are half that of 1st weekend. Studios know this.
We here like to talk about "But what about international box office and DVD sales?" Those can certainly help, but they have to help to an extraordinary degree, from what I've observed casually. Like, you'd need to ultimately return, like, 60% of the budget domestically, and then something like 200% of the budget from DVDs and international box office -- and even then, your initial budget would need to be relatively low.
Otherwise, why would the studio greenlight a more expensive sequel, when they can spend that same money on something they know will make bank in opening weekend, domestically, and will get those DVD sales and international box office sales?
I'll put this another way. I deal with this mentality in a limited sense in my own business. My clients don't always have this attitude, but sometimes the more entrepreneurial ones do. The basic line of thinking is not about how much money you did make, but how much money you could have made. People look at the "could make" number, and the treat that, mentally, as the number. When the actual number falls short, they don't look at the glass as half full; they see it as half empty. In other words, they think about all the money they "left on the table" that they could have made, and therefore view their venture as a failure.
That, I suspect, is how studios operate. They greenlight a film and they have a target number in mind. If they exceed that number, they're delighted, and you may see a sequel. If they meet the number, it depends on how franchisable they think the film is. If the number falls below the target, it doesn't matter how much profit they did make. What matters is how much profit they didn't make, and it's by that measure that the film's failure is determined.
When you then blow that up to an entire business for a whole studio, you've got some serious budget considerations. Because, of course, studios make more than just big-budget superhero films or licensed properties. They're also funding their indie label (e.g. Fox Searchlight). They're funding comedies, and romcoms, and serious dramas, and obvious Oscar bait, and family-friendly films, and so on. The big budget films could well end up paying for those projects. I'd bet sometimes they end up paying for projects within the same "department," even, such as when a big budget sci-fi film ends up supporting development of another big budget sci-fi film. So, when a film like Tomorrowland tanks, the studio has to tighten its belt. What's gonna be the first bit of fat trimmed? The expensive sequel from the "modest" success, when that money could go towards developing a new franchise, or towards supporting an existing one that's already a proven success.
