The first reason, is weight. The A4 isn't exactly a tiny car to begin with, and with the added weight inherent in a convertible(extra bracing and such), the last thing you need is several hundred pounds of extra driveline components mucking things up even further. The second, Audi assumes that a convertible is, by and large, a fair-weather car. And, since you don't really need AWD when you're cruising around in the sun, it was omitted. Thirdly, and most importantly, from the looks of things, it'll only have the multitronic(unless I'm underinformed), which won't work with quattro.