DSL was specifically designed to take advantage of the existing telecommunications infrastructure. Designing it around an expensive, niche market technology would have completely defeated the purpose, which is an accessible, high speed connection, at a reasonable cost.
Paulson...
ISDN never caught on because it was way too expensive for the average home user. No business in their right minds would rely on DSL or cable, though. Frame relay based solutions are MUCH more reliable, but are also much more expensive. Someone who just wants speed can deal with outages. Someone who absolutely depends on their internet connection can't. They're aimed at different markets.
Viper GTS