- BTC tends to fluctuate a lot. I could buy a pizza with it today, or a car tomorrow, or a house in a week. Therefore, I am going to think twice about buying a pizza today.
- Tax implications of BTC are complicated. If you have never owned BTC, and this year decide to buy it to experiment with it, now you have additional paperwork to file about it. If you buy it via a BTC ATM, or some other cash method, you now have to keep track of all the records. (Presumably, places like Coinbase, etc. do this for you, I haven't checked). This is not insurmountable, but that's different than taking $20 out of an ATM and buying a slice of pizza.
- BTC is hard to acquire. Yes, yes it is. Coinbase requires a multi-day verification process.
- BTC is hard to keep secure. To do this properly, you have to know how to create an encrypted cold storage wallet and how to keep your computer safe so that when you unlock it, someone doesn't grab the wallet while you are using it. This is hard. It's so hard that intelligent people with domain knowledge get it wrong. I don't expect someone who is not a "computer person" to just get it. Coinbase and such are great for the "not computer persons", but are not secure (as in, you need to use 2 factor auth + strong password, which most people do not do; it's also not secure in that if someone does get into your account, you are done for. There is no reversal/insurance).
- BTC is weird when you are spending money. With fiat currency, I go on Amazon, punch in my credit card number, hit "buy" and viola! my order is all set. Theoretically, my credit card can get rejected later, but in practice it doesn't happen; if it does, I am familiar with the process. With BTC, I am suddenly shown number of confirmations, have to wait for minutes to know if my order went through, etc. Imagine buying Google IO tickets with BTC: by the time my confirmation comes through, someone else already paid with a faster method and got my reservation. (I know they recently changed it to a queue/invitation system, but my point stands for lots of purchases where the quantity is limited).
- BTC is weird to spend, again. If my BTC is in an encrypted cold storage wallet, as it should be, I now have to defrost it, decrypt it, push the BTC somewhere, then wait. Oh, and for some modes of spending, I then have to transfer BTC again, and wait again (localcoin for example).
So, where will BTC go from here? Well, I think it'll continue being a specialized currency. It's not great for buying pizza, and I don't think ever will be. It is great for transferring money in/out of restricted jurisdictions, or from person to person without leaving an obvious trail. That part works well. If you are a Snowden-type whistleblower and need your friend to get you money, this is as good a way to do it as any. Or if you are my grandfather who cannot get his money out of Ukraine due to the banks "not having enough USD on hand" to wire his balance to a US bank. Or if you want to make an anonymous donation. BTC is basically a faster and more secure version of stuffing cash into envelopes and mailing it.