Say you used one credit card for all your transactions and say you used it at Target this past holiday season. Then say that you were one of the folks whose credit card company said: "We're cancelling your current card and will send you a new one." What, exactly, would you use to pay for things until the new card arrived? That's one reason for cash.
Say you're dining out and say that the power goes out, or simply the credit card processing companies are down when you try to pay. What do you do? If you actually carry cash, you have a significantly smaller problem.
The problem with credit/debit cards is that they are entirely dependent on services being up 24/7. Bitcoin has a similar problem but with less opportunities for problems. Cash on the other hand (and I'm talking about the paper version not the 1s and 0s that banks count as 'cash') doesn't rely on anything of the sort. There are too many points in the credit card transaction process where a point failure can cause significant problems.
So, we still use cash because it can be used when credit/debit cards cannot. Assuming, of course, that the recipient knows how to count (and make change in the case of retail outlets).
Then there are those who prefer that there not be an electronic trail of all their purchases. With the Government trying to pin the 'terrorist' flag on everyone from those trying to be self-sufficient to those trying to prepare for a potential catastrophe, a lot of those folks prefer a cash purchase with no other records than their receipt and their name uninvolved and unrecorded.
On a side note, has anyone else ever realized the humor in FEMA advising everyone to prepare for X number of days in case of a natural disaster while other branches of the government say that those who do things like that sort of preparation are potential terrorists? It's almost enough to make you cry, unless you're already cynical about government in general.
Thralen