Problem with more digits on a pin is you need compatibility.
Yes ... my father still has a 4 digit pin with CIBC.
I'm with Desjardins for over 10 years, always had a 5 digit pin.
If you give out 8 digit pin, too many people will forget.
For example in the UK, you'd need ALL banks and building societies to switch to using longer pins which would require new bank machines or at least a firmware upgrade, plus all the database infastructure would need upgrading.
That would cost BILLIONS. As they say, 'if it ain`t broke, don`t fix it'.