Can someone explain the importance of Nirvana to me?

I like their music, but they are constantly being called one of the greatest bands of all time, and they have 3 albums. On Rolling Stones' list of the greatest artists ever they rank higher than musicians like David Bowie, John Lennon, Michael Jackson, Madonna and Johnny Cash. I'm not really fan of any of those artists, but I understand their importance. So can anyone help me out with this?