It's because audio still isn't well supported on the web:
HTML5 audio has glitches, high latency and bugs, and isn't supported well on mobile
Web Audio API only works on Chrome, but has different tradeoffs, like you don't seem to be able to set the position of a currently playing sound
PhoneGap and AppMobi provide other APIs to play audio, but they are limited to simple "play sound" functionality.
Basically, there's still no widely supported decent audio library for HTML5, so we're stuck with limited features for the time being.