It's about the total amount of revenue a property can potentially generate. The Box Office Mojo list of franchises doesn't quite capture that.
Lily & Stitch, for instance, doesn't show up on that list. But the characters sell tons of merchandise to this day, as of right now there are 184 results for "Stitch" on ShopDisney (more than Cars, Monsters, etc.)
The key thing is that with any property, Disney corporate has to strategize how best to support that property in order to extract the max amount of revenue possible. There's so much to consider.
Would a Stitch attraction increase merchandise sales? Maybe, maybe not. It's possible the character is already reaching its peak and a new attraction won't meaningly affect those numbers. Or is an attraction is needed to sustain these numbers? Has a lack of an attraction hurt these numbers? Stitch sells more in Japan, is that a cultural difference or because he has attractions in both Adventureland and Tomorrowland?
There's a fair amount of surveying, forecasting, guess work, creative ambition, and personal taste that goes into all of these decisions. It's not as simple as Cars is the biggest franchise without an attraction, so let's add an attraction. Or Muppets doesn't sell merch anymore, let's get rid of the theater. Those decisions might make sense (and probably do), but there are multiple factors at play.