I think the key reason why cashiers give out 2 $5 bills instead of one $10 bill is pretty straightforward. This has been the norms since the early-90s, if I recall. Back in the 80's, there were slightly more $10 bills in circulation than there were $5 bills. Since around 1990, the number of $5 bills in circulation overshadowed the $10 bill and has been this way since. The $10 bill has been in decline since the 1940's, if I remember reading correctly somewhere.
Cashiers do give out 2 $5 bills instead of just one $10 bill because it's later in the day, and at that point they had received so many $5 bills over many hours being open, plus they need their $10's in the event that a customer pays with a $50 or $100 bill. The larger bill a customer pays with, the larger the denomination a person receives in his or her change. Plain and simple. They will only receive a $10 bill in their change if they receive at least one $20 bill in the change. If they don't receive a $20 bill in their change, then two $5 bills comes into play.
I get frustrated easily whenever I get two $5 bills instead of a ten, even more so when the cashier does have a ten in their till. But the 80's are over. Welcome to the 21st century - where $10 bills are quite a lot less prevalent, and the $5 and $20 bills rule our currency. I've been seeing two $5 bills in my change a lot more since the fall of 2011 and I also noticed since 2000 that it seems as if every 3 years, my province loses one new $10 bill prefix. In 2002, I used to see 5 new prefixes. Now, I only see 2 new prefixes. Yes, my friends, it seems as if, in my region, the $10 bill is on the way out. I have a feeling that, come 2020, or sometime in that decade, all the banks in my region won't be ordering any more $10 bills.
But I agree, I rarely use cash nowadays. I'm so frustrated over not seeing any $10 bills in my change any longer that I rely on my trusty debit card for payments now. Plus my interest in cash is dying as well.