Good ideas come in cycles. At that time, though, 50% of the world wasn't mobile-only, and there were no smartphones. We think this is really relevant right now.

The phrase I've heard is that being early is the same as being wrong. Trying to make Burner in 2003 would be like trying to make YouTube in 1996.

Any conjecture as to why?

Assuming you mean why it's relevant now, not why mobile adoption is up, the mobile number travels with you at all times and also is more heavily used for SMS, which is becoming an increasingly standard way to interact with people. By definition this means a greater and greater percentage of the people you interact with are running through your mobile phone, meaning more carrier minutes, more annoyance, and more stalker risk if people have the number that you don't want to.