My reasons are:
Security- No other payment system is even close to the security of bitcoin. Only you control your coins and there is no need to associate them with your identity. It is this association that makes credit cards so vulnerable. Each time you use one you are sending all the information needed to rob you. And you send it to dozens of people you don't even know.
Price- You may think credit cards are free, but that is just not true. Someone has to pay the 3% and it will be passed on to you, the buyer. Go to a place like overstock.com and you will notice that if you pay with bitcoin the price is 3% lower. That could be quite a bit of money over time.
Speed- Credit cards may seem fast but in reality it takes days or months for a charge to clear. With bitcoin I can buy something and the seller can ship or provide services within minuets or hours. The seller has the money, not just a promise of money.
Unassailable universal wealth- With other payment systems you are a visitor and subject to whatever rules the processor feels like imposing. These systems are backed by currency that is also beyond your control. You can use it when, where, and if you are permitted. It is not your money.
With bitcoin I decide who I want to pay, regardless of where they are. I decide the terms with the only other party involved. It is truly a free market and no one can do anything to coerce or dissuade me from doing what I want.
Come on dude, you have to know those reasons aren't really true. At least not yet.
Security is a relative construct. Bitcoin is no more or less secure than other methods of payment if it's used incorrectly. More people have been robbed using exchanges than I like to admit. Bitcoin virus attacks on not only business computers but personal computers are commonplace. Banks get hacked too but they replace the lost customer money with fraud protection. Fraud protection is what you're paying for in the service fee you pay at a merchant terminal. You also currently, and for the foreseeable future, must involve the current financial system at some point to make a Bitcoin purchase. Sure, I suppose those people that choose to convert their money by purchasing expensive mining equipment or using LBC to acquire bitcoins aren't involving banks, but that's impractical for most people.
The cost of goods and services are moot. The price isn't less when you consider exchange fees and fraud loss. As I mentioned above, the 2-3% fee pays for fraud protection that I sorely wish I had during the Bitcoinica and exchange hacks where I lost more bitcoins in less than two years than I have ever lost in fiat in 50 years.
Bitcoin is slower, period. The reason transactions in fiat banking aren't reconciled immediately is because of that fraud protection thing I mentioned earlier. The charge is satisfied and reconciled only after all parties are in agreement but for customer purposes it's immediate. Bitcoin uses the VERY SLOW process of trusted third party escrow for that job.
Banking is a free market too dude. I only use credit unions for banking because they offer the services I want and need. Bitcoin can't compete and no current third party Bitcoin service can currently come even close to the offerings of a credit union. Also, as long as Chinese miners provide the security for Bitcoin not only am I not in complete control but my country isn't even in control.