In addressing the question about the title associated with the dramatic performative art that, in English, is known as Beijing or Peking opera, Joshua Goldstein writes: "The simple answer, then, according to Qi Rushan—probably the most influential, encyclopedic, and prolific expert on Peking opera in the twentieth century—is that Peking opera (whether written as jingju, jingxi, or pingju) is merely a label pasted onto dramas performed, for the most part, in a style comprising several dozen melodic themes otherwise known collectively as pihuang" (Drama Kings: Players and Publics in the Re-creation of Peking Opera, 1870-1937, p. 2). Goldstein goes on to discuss that the form of performance we know today as Peking Opera (and many of its titles) truly came about in a post-colonial era and, therefore, requires a great deal of historical work to navigate its colonialist past so one might its examine the pre-colonial traditions, aesthetics, practices, etc. So, to answer your question, it's really more about what European/Western colonial individuals thought about its form and aesthetics. The reason for calling it "opera" (as opposed to "theatre" or "drama") was due to its formalized structure that came through in the specific styles of vocalization and in the performative significance of makeup, costumes, gestures (i.e. actions), and character types. Most of these were also present in Western opera (though realized quite differently) and thus the Chinese version was considered a "high art" like Western opera. For reference and more information on Peking Opera, I'd also recommend: Peking Opera by Chengbui Xu.