1) When people are making money off of something, they protect that method of making money, and evolve if necessary.
2) There are intricacies to our currency that might not seem immediately obvious. People depend on the discreet nature of physical currency - it controls the entire world of corruption (a lot of movies like to use diamonds as black market currency, but in reality its too difficult to rule out man-made diamonds these days, or to quickly evaluate the worth of a lot of diamonds). Corruption does not occur solely among gangsters, but all manner of people - from the most prolific bankers to the most "reputable" politicians. Digital currency can too easily be traced.
3) The government has made little effort to create its own digital currency, despite the fact that it would save tax payers billions, negating the need for the IRS, Mint, and reduce the need for the FBI and Secret Service (which monitors counterfeiting among other things)...and the corruption associated with physical currency. You might encounter more red tape and inertia than you anticipate -- if not now then when Visa lobbies its favorite candidate to create laws that work against you.
4) Third world countries are not a great target for digital currency IMHO, unless they have prevalent technology and understanding of it.