A simple answer is marketing and maturity.
When a huge corporation officially backs Bitcoin and publicly makes it obvious that Bitcoin is a big player (for example, placing a Bitcoin symbol or "Bitcoin accepted here" banner on the main site or checkout area), I have no doubt it will become way more publicized and used than it is now. I'm annoyed by companies saying they support Bitcoin, but it ends up being something that's just left hidden someplace that no one can seem to find. Just look at sites like Newegg (I think they support Bitcoin? I'm not even sure. Maybe, it was just allowed once at a promotion.). If only they place the Bitcoin amount along with the USD, just imagine the amount of interest generated for Bitcoin. People will then find out through their research or through other people including friends and family who did their research or got their info somewhere else that there are a load of benefits to using Bitcoin. Three particular attractive benefits are instant transactions, very low fees, and immediate and easy usage.
For most people, Bitcoin is interesting, but it's boring, and no one really cares for it. People are comfortable already using cash and their credit cards. Why would they even consider this when they are already happy using what they already have? The way to break this mindset is to make it the next hip thing. That means famous youtubers and large companies promoting it as well as respected and known writers/columnists writing big and continuous stories for it. By doing this, people will begin to recognize and accept the benefits it provides over fiat money.
Bitcoin shows that it is not yet mature enough. It seems most people who hear of Bitcoin read stories about how someone hacked and stole a large amount of Bitcoin that is worth thousands of dollars. People don't yet trust it because it hasn't been around long enough and hasn't really been seriously supported by big players. Everyone is comfortable using fiat money because they have been exposed to it their entire lives. They trust it and feel comfortable around it. National currencies have been around for generations. It's their way of life.