Hello Drsparis. You may hear many different reasons, here are my comments:
-Convenience - many people use their (noicy) PC as their main source; at work and at home. USB is simply easy. My PC's have fans and that ruins that option for me.
-Asyncroneous USB seems to solve most of the initial problems with USB (search on google or here for lost of detailed explanations)
-Still a lot of people use Sonos or similar which uses spdif. That's easy too.
-Dedicated audiophile sources (like NAD M50, Linn etc) are too expensive for most people.
-People are afraid that dedicated audiophile sources would be out of date within a year.
-Once portable sources such as the Astell & Kern type become even better (and better priced) they may take a larger share of the market since having ones complete catalogue everywhere is easy too...
In any case everything currently points towards the DAC as the main "hub" in a modern HiFi setup. Personally I think it's very interesting, especially the type thats completely regenerating the signal like the NAD M51 and PS Audio Nuwave DAC. In the future I hope to see more clever signal processing options in this kind of DAC which can hopefully improve on the horrible mastering typically used in our present times...