My best guess would be that generally singing/music shows are popular, and "I am a singer" has some artists that are well liked in China. It could be a side effect of all the talent shows, that sparks life to the dream of the average person becoming a start , and if there is any place in the world were riches for limited effort is sought after then it is China. But as "I am a singer" does not have this aspect, all I see is people singing different songs, and sometimes even killing otherwise nice music as their talent does not match what they are to sing. 

Well, did you? Did you watch it? Huh?

It's easier to sing than play an instrument. Not to be critical, but singing requires little or no equipment, you simply sing. How many people hear a good song, depending on their musical taste, and try to emulate it. How many singing shows are there in the U.S.? It seems popular everywhere, and it gives parents a chance to say "There's my kid!" It obviously gives one a sense of accomplishment to sing a song well.

I have never heard of it...