Olympic medals and in particular Olympic track medals are not particularly important in cycling. Track is fun, but a backwater that few people really cares about. Road is where pretty much all of the money, talent and audience are. Britain isn't one of the countries where there has historically been a mass audience for cycling and a lot of the casual audience there has been introduced to the sport through the British Olympic medal machine. Understandably this tends to leave many with a very skewed impression of the general pecking order according to fans everywhere else in the world. Track probably ranks somewhere after mountain biking and cyclocross from a continental point of view, Chris Hoy isn't remotely famous and British cyclists important breakthroughs in recent years have been on the road.
On the road, the Tour de France is the most important race you can win. But there's a TdeF every year plus two other Grand Tours and six very important one day races and various other races of some note. Winning a single Tour de France is a great accomplishment and life changing for a cyclist. But quite a lot of guys win one Grand Tour and not a huge amount else of importance - Sastre and Pereiro, the two guys I mentioned above, both won the TdeF once and now, not all that much later, are almost forgotten. Cavendish, on the other hand, has won and won and won. He has won the World Championship Road Race, Milan San-Remo, 30 Tour de France stages (more than everyone in history bar Eddie Merckx), Giro and Vuelta stages and countless smaller races. He's been the best sprinter in the sport for a decade. When he had a couple of "bad years" he was still winning fifteen races a year and the issue being debated was whether he was still the very best or just one of the three best. This year there's no argument again, he's the best. He has years to go and he's already generally regarded as the best sprint specialist who ever lived. He's part of the pantheon of the sport. In thirty years time, the question cycling fans will be asking about the top sprinter of the day is "but is he as good as Cavendish"?
You have to have at least a little background knowledge of pro cycling to understand that Cavendish is a great. While the less you know about pro cycling the more impressive Wiggins career sounds, particularly if you come from a country where winning a heap of track medals makes him "our greatest ever Olympian" rather than anywhere else where that's about as relevant as his prowess on a BMX. That's not to say that Wiggins hasn't had an excellent career. He has had. But he's not a great of the sport, unlike Cavendish.