Because they have very stringent qualifying requirements.
When I first signed up with them I had a lousy $200 weekly limit.
Got it bumped to $500 but got tired of dealing with them.
For example, when I moved, Ichanged email address, they wanted an explanation, same thing when I wanted to use a different bank account, they wanted a reason before they'd update the account info.
What annoyed me the most with them was if I got a $USD payout from a poker site into my Instadebit account and wanted to send a $CAD deposit to Pinnacle they wouldn't convert the currency, they required me to withdraw the US to my bank then they would withdraw the $CAD from my bank account. Plus the week to 10 day waitng period before you could withdraw once you made a deposit via instadebit.
I converted everything over to moneybookers and no longer use instadebit.
Instadebit
a real pain in the ass!
Moneybookers is much more convenient for moving money around.