Because Stamford Bridge is less than half a mile away from five tube stations which are connected to three different Underground lines, and because it's situated in a capital city that knows how to move massive groups of people very quickly. The Etihad is stuck on the edge of a council estate and next to about three tram stations, all of which are on the same bloody line anyway. Metrolinks hold about 10% of your average Underground train and they only turn up once every 15 mins as opposed to there being one coming in every time you turn round in London. If you get out of Stamford Bridge a bit late after a night game you'll still make it home for 11 - if you get stuck at the Etihad after full-time it could be the next day before you're out of the area, unless you're willing to walk a mile back to Piccadilly. The two are not comparable.