I like the idea of being trapped inside of a bubble. I've found it difficult to fully articulate why that is. I remember being interested in the concept in some capacity since a very young age, like four or so. I'd feel really weird whenever a character in a cartoon or whatever would get trapped in one, I'm not sure exactly when it crossed over into fetish territory but I think it was around middle school. I've given a couple of different reasons for why I find it so appealing over the years, but I think they're all a result of me trying to rationalize something that can't really be rationalized, especially since most of them (bubbles are squeaky and shiny, bubbles are like a less extreme version of bondage) are based off of fetishes that I developed because they were downstream of this fetish (latex, bondage).
If I had to take a stab at it now, I would say that it's appealing almost as a form of "hobbling" rather than outright bondage. Someone trapped in a bubble could still roll their bubble around, maybe they could push it slightly if it's the kind of bubble that floats, but they're still totally unable to do anything actually useful. I think this is also why my favorite kind of "normal" bondage is petsuits, it's the same kind of appeal. Essentially being reduced to a "pet' or some kind of object for whoever did it do you.