Which even amplifies the observation. Not only is the process of proving a theorem more complex than it looks in retrospect, but the whole process of deciding whether a theorem or even a whole field of math is even useful or not can be driven by chance over a time span of decades. What is now "topology" was kicked around for many years with many approaches before a general agreement that open sets formed the most useful basis (heh). Advanced textbook authors regularly have to cut topics from subsequent editions of the book, because what seemed promising 30 years ago turned into a dead end. So math is far from the discrete list of topics and answers that it seems to be when you're picking out classes in your syllabus.
For anyone interested this seems to be a reasonable summary: https://en.wikipedia.org/wiki/Controversy_over_Cantor%27s_th...
According to Wikipedia this particular theorem was proved almost immediately (but not published) by Dedekind, and then 10 years later proved by a 19 year old student in Cantor’s seminar. It’s not clear whether more than a handful of people cared or worked on it in between.
As for Cantor being a leading mathematician of the day: most of the other mathematicians of his time regarded this whole mess to not be mathematics at all. Poincaré called Cantor’s set theory a “disease”. It wasn’t until several decades later that a broader group of mathematicians decided that roping in set theory allowed them to conveniently hand-wave away (“oh, the set theorists will take care of that part”) a lot of thorny questions, letting them get on with their work as they had before, but now with an deflective answer whenever anyone asked such questions. ;-)