If you look at it as Currency, it is difficult for an average person to understand it because it is different.
Bitcoin, being a different kind of a digital currency, it breaks down conventional thinking in many aspects. No central regulatory body. The concept of mining. The open-ledger (blockchain), the way miners are rewarded, how transporting value is determined by its packet size and not value, the hash rate, the concept of limited supply. The 100 Million pennies in a Bitcoin, etc.
Its difficult enough for people in the money space to thoroughly understand Bitcoin (we all say we understand it, but do we really?).
For a common person, its an overkill. Its like trying to explain the concept of TCP/IP, Networking, Routing, the Internet for someone who wants to use email. Overkill.
I don't think it's difficult to understand at all. What I think is that people needlessly over complicate it with all the buzzwords like blockchain, decentralized, peer to peer etc and even more people are scared because of all the demonization in the media (oh, it's a scam and ponzi and drug dealers use it). Most people don't even know that fiat money refers to the money in their pockets so I'm not sure why we should expect them to know the economics and technology behind how bitcoin works right down to the code because they don't need to. Do you need to understand the complexities of how a computer or the internet works to be able to use it? Absolutely not. You turn on the computer and click a few buttons. Easy as pie. Do you need to know the mechanisms of how a petrol engine functions to be able to drive? Not at all. You put your foot on the gas (and brake when needed) and turn that circle thing in front of you and you get from A to B. Magic. Same goes with bitcoin. Bitcoin is very simple to use, especially with the ease of some apps today. I've seen someone mention this before that using the blockchain.info app isn't much different than using paypal and might even be easier and more secure. Scan a QR code, put the amount of bitcoins you'd like to send in (and use the dollar amount if all the decimal points confuse you and click send payment. Bingo. What is complicated about that? If you can use online banking then you can use bitcoin. One other function of bitcoin other than it being decentralized money (oops, evil buzzword again) is that people can treat it as an investment, and it's one that is very easy to get into. Most people don't know how to invest in the stock market or whatnot so a lot of them are now getting into bitcoin instead, which is a good thing. Just don't overly complicate bitcoin to newbies and explain it by comparing it to other things they're already familiar with. If they can use banking apps and paypal then there's no reason why they can't use bitcoin, so maybe just explain why bitcoin is good or at the very least different to current monetary models.