IMHO - for the same reason many people switched on their own. A standard double gives me many gear combinations I (and I believe, 95%+ of the riding public) I will never use. Even with the compact, I can't remember the last time I was on anything bigger than 50/15, which let's me cruise (with a brisk tailwind) comfortably at 26 -27 mph. I stay on the 34 down to the 13 cog (11-25), and if there is enough breeze I go up to the 50/17. As an example, the entire ride below was on the 34 ring (almost NO wind).

http://ridewithgps.com/trips/977859

Why do you think compacts have become so prevalent? Or do you question that they have? I have seen compacts on many tri bikes as well.