Same reason there's so many CF bikes with sloping TT's. With a sloping TT, you can fit more riders with set stock geometries, save a slight amount of weight, and allow for a marginally stiffer frame.... or so they say. This way manufacturers can branch out to a larger group (i.e. 'the masses') since custom geometry jacks the price up to only those that can afford to replace it. Market to the masses and you increase revenue exponentially. I bought a stock Ti frame when Lynskey came out with their houseblend to avoid the extra charge (like $2K more) for custom and it's been a great frame for the past 4 years.