Because I'm looking for a "performance" car that is also suitable for commuting. I am not saying that the A3 184Q is better than an S5 or RS5 in out and out speed, but it does have other benefits that push the balance back in it's favour (i.e. low running costs with relatively good performance yet delivers power very low in the rev range - which is where it needs to be IMHO).
Whilst I can totally understand an S5 or RS5 as something you can have fun in, whatever I end up with has to be very practical too. Driving 60ish miles a day on A/B roads, I need something that is powerful enough to have some fun in when I am able, I am also looking for something that either gives me goose pimples every time I drive it (so I can justify potentially 3 gallons of fuel A DAY which is over 2x what I currently spend) or is economical enough that I can afford to buy something else other than fuel for the car!
Fundamentally, I'm comparing them as I want something "better". I was expecting to come away from the test drive thinking "how can I live without driving an S5/RS5 on a daily basis". I didn't, but that isn't to say they are a bad car. I would love to have either as a fun car which I could use when I wasn't worried by fuel costs, but that isn't practical....
I'm rapidly coming to the conclusion that I either stick with what I have or look at the 3l quattro diesel. Or a Beamer (which seems to be the only other practical "performance diesel" alternative). I think I am coming to the conclusion petrol is out as the cost-per-goosebump (tm) is probably too high.