USB polling mechanism

Why does USB use a polling mechanism instead of an interrupt to detect the device?

I've read here on Stack Overflow that if the event is

1. Synchronous (i.e. you know when to expect it within a small window)
2. Not Urgent (i.e. a slow polling interval has no ill effects), and
3. Frequent (i.e. majority of your polling cycles create a 'hit')

then we can use polling. But in case of USB, device detection is not frequent, so an interrupt mechanism would be better.

Thanks for your replies,

Zaheer