As a few people have suggested many of the old B/W are classics because they have stood the test of time. Very few of the best movies on the BFI Sight and Sound list are dated after 1970 even.
'Citizen Kane' was made in 1941 and is often rated the greatest movie of all time and it is, of course, in B/W. It's seen as a classic now but it wasn't at the time and didn't win the main Oscar. I guess in terms of film history it takes a while to get critical and popular recognition, so it's only a matter of time before some of the modern colour films will be regularly seen as classics. 'Apocalypse Now', 'The Godfathers I/II', 'Bladerunner', 'Fargo' are probably already that.