Yes, this is the only relevant question w.r.t Pay at the Pump. If there aren't significantly more stations now than last year, then the 54% vs. 70% figures are more likely attributable to people not wanting to pay high gas prices out of pocket.
There might be another explanation.  I used to make debit purchases at supermarkets and indoors at Sheetz, a big gas/convenience store in the PA/MD area.  Soon after I opened an account near my new office, my bank began to charge 50 cents for each *debit* transaction.  So if I go to Giant, Weis or Sheetz and buy something, and tap in my ATM number, there's an extra charge on my statement.  If'n however, I choose *credit* transaction, there is no charge (to me).  Giant and Weis aren't dumb, their machines practically beg you to type in an ATM number, but I soon learned to get around that.  And at Sheetz, I spend so much on gas outside that I don't automatically run in and buy anything anymore (their subs used to be great), so less chance of a debit transaction at the counter.

Plus, who carries enough cash to pay for 40 or 50 dollars of gas?

OK, maybe. But again, did this bank policy change between 2004 and 2005 for most people? It's your last comment that's exactly the point. No one carries that much because no one wants to spend that much. So they pay with the credit card.
I think so.  The fellow working next to me said he has seen local news articles on the change in policy, and that it is widespread around here.  

And I should have noted that the money comes out of the same place, my account, with either credit or debit transactions.  The only difference is who pays the transaction fee.  

I did use a classic credit card for gas last year, when I was a self-employed builder.  I hated to do it, but I hadn't made any money yet.