Yes, there is a fundamental reason we cant have more than one sheet in a campaign. It's because - in a sense- there is only one sheet in the campaign. All the different characters in the campaign are using the same sheet. Not the same type of sheet - the exact same sheet. They are connected. When sheet designers set up their sheet, they have to follow certain rules that dont exist elsewhere on the web. There's a type of element html uses called an id. This can be used to layout sheets, and different parts of the page can have different ids. But no two elements can share an id, they must be unique. We cant use ids on roll20, because if one sheet has an id, they all have it, and thus it's not unique. A change to one id would affect every character in the campaign identically. Imagine there is only one sheet, but the character displayed on it changes. Like the character sheet is a book, and each character is a page in that book, and you flip through it whenever you look at a different character. This is a simplification, it's not exactly how things work. But it's a good way to conceptualise it. This is why you cant have different sheets in a campaign: its in the foundations of how roll20 handles character sheets, and it would take a massive rebuild to enable different sheets. It's easy to imagine a way to make such a feature work- have each sheet in its own sandbox, isolated from the others. But saying that and doing it are two vastly different things: the technical hurdles to achieve it will be immense. There's no real incentive to the devs to work on such a feature, when it would be so costly (in devtime and resources) and when it is needed by a tiny fraction of the userbase.
