Maybe because Alfas of old used to be some of the best looking cars of the time. Not the most reliable things, granted, but still stunning machines.
And maybe because Alfas of today are both very good cars _and_ also damn sexy.
Check out google images for Alfa 147, Alfa 159, Alfa GT, Alfa Spider, Alfa Brera, Alfa Mito and Alfa 8C.
Probably the only automobile marque of today that hasn't a single dud/ugly duckling in its line up.