You guys are really abusing the definition of alternative. Someone said Boston. Alternative music is Pixies, R.E.M., Television, Sonic Youth, even Nirvana, Radiohead. Green Day and Oasis are alternative, but barely. Neither are the best alternative band.
There's also some blind love. I'm actually fans of both Green Day and Oasis, but neither are exactly the most artistically credible bands of all-time. Oasis blatantly rips off the Beatles; Green Day, the Clash. The average person does not know the melody to Geek Stink Breath, Walking Contradiction, Nice Guys Finish Last, Hitchin' A Ride, Redundant. Live Forever or Supersonic are about as well known. It takes dedicated fans to really care about the lesser known hits.