Try watching 12 hours straight of it - it will drive you insane, especially traditional productions that are usually performed in no-longer-existent dialects. Reading Chinese opera is far more pleasureful than watching it, I'm afraid - it is rather sad, but that is probably why the operas that seem to be most studied now are known for their literary quality rather than their theatrical quality.
Not saying Chinese opera is good or not (though you would need to specify which kind, as Chinese opera is different by tradition in every city) but that 99% of Chinese people under 50 would probably agree that they will never desire, or be able to sit and watch a whole opera from head to tail. in addition to this, most would go insane in 5 minutes.
The general form Chinese opera takes now on the mainland is various programs broadcast for mostly old people on TV (in various languages) as well as school trips or short videos showing the most famous 5 minutes of any opera (which most people cannot even stand).
That being said, I will say the reason is simple - people don't like the music, and the emphasis has traditionally not been on the music, but more on the performance. In a world with cinema and developed effects, a juggler is no longer a ticket seller like it used to be.