I think there were multiple issues around Cats that shouldn't apply here. People have mentioned Wicked being light on plot, but Cats barely has a plot at all and the decision, as
@nathanspears says, to use CGI rather than costumes was a hellish miscalculation.
Plus there's the fact that Cats' heyday was long behind it by the time they got round to the film, so it had less resonance with a modern-day audience. Wicked is still running on Broadway, as well as in London where it's the longest running musical that hasn't yet been adapted into a film (Phantom Of The Opera, Les Mis, Mamma Mia) or wasn't already based on one (The Lion King). Not to mention the number of successful tours it's had over the years, plus the songs gaining extra popularity through Glee, covers, talent shows etc. With that level of name recognition plus the Ariana diehards to rely on, it should be a recipe for success but
only if it's done right. The decision to split it into two films is already ringing alarm bells on that score.