The OP said slow adoptive rate not slow value growth.

BTC (and other cryptos) have a slow adoptive rate for a few reasons. Here are a few:

1.) It is still really confusing and not newbie/computer newbie friendly. Go ahead, try to teach a 50-year-old about bitcoin in 5 minutes. You can teach a person everything they need to know about a credit card or checking account in 5 minutes, easily. Bitcoin takes hours and many follow-up sessions.

2.) It is still plagued by scams, frauds, and technical issues. The average joe can't be expected to constantly follow forums and reddit threads to learn of when some new exploit is discovered and the wallet needs to be updated right now or you risk losing your coins.

2b.) Updating the wallet properly (including backing it up beforehand and in some cases re-downloading the blockchain) is confusing and scary to newbies and the not-so-computer-literate.

3.) There still isn't a compelling reason for anyone to switch from primarily using fiat to primarily using BTC. It's generally confusing, it costs money (fees and such) and it is risky to acquire, hold, and use BTC (and other cryptos.) The majority of people involved right now are all investors in one sense or another, who are willing to take the risk only because they believe they will make a lot of profit.

3b.) If you disagree with the above, ask yourself - if you were to convince someone to start using bitcoin for the majority of their financial transactions, what argument could you make, other than "it will go up in value so you can make money that way" ... ?