Basically your answer lies with either:
a.) Rivers demoed the song (i.e. POW) and although good, didn't think it sounded like something for Weezer.
b.) The song was put aside because there were better demos (i.e. Buddy Holly which was written around the same time as Chess) at the time that deserved being on a record more.
Rivers has written, as you can see, a bajillion songs.
There's also a torrent floating around that's got nearly all the demos and such.
as for info, the Weezer Recording History right here at weezer.com is pretty great reading