I never got why 4 digit PINs were so common. Not only does requiring more digits make it harder to guess by brute-force, it also prevents people from making frankly stupid PINs like the date of their birth year.
I'd hazard a guess that asking for an 8 digit pin will see lots of people just repeat the first 4 digits again.
As others have said, increasing the length would do little to increase security in most cases since the usual response will be to repeat digits. Plus the current system seems to be working pretty well empirically.
This reminds of the password rotation policies that some companies insist on using, despite the evidence that they are a waste of time. In practice they are an "Increment the last digit of your password" policy.