No manufacturer has been making bicycles longer than Bianchi.
That probably gives them a bit of a start in the "icon" stakes.
I think the other factor that contributes to their street cred is their prominence in what many regard as the "Golden Era", when the likes of Coppi and Bartoli, Anquetil and others were such revered figures.
I would regard Peugeot and Colnago as iconic as well and no doubt everyone out there has their own idea of what constitutes an icon.
I know the world has moved on since then, but I don't see Trek or Spesh as iconic brands, albeit they make excellent bikes.
(I have a Trek and a Bianchi)
Cheers,
pj