March Madness Non-D&D Blog Challenge: Day 5: What other old school game should have become as big as D&D but didn’t?

What other old school game should have become as big as D&D but didn’t? Why do you think so? 

I've never actually played it but probably Runequest.  It had solid rules for play and a great non-traditional (non-medieval) world.  Back when I read Alarums &  Excursions I found the discussions about RQ engaging and fascinating.  Sadly I was low on spending cash as a kid and all my purchases were for my beloved Chivalry & Sorcery.   Speaking of C&S, I can't really propose it as the game which should have made it big, despite my fond memories of it.  It was too fiddly and lacked "fun".