Because to complete a bitcoin transaction it needs to be accepted on the block chain. That incurs a relatively high flat fee (somewhere between $5-$50 dollars

Because to complete a bitcoin transaction it needs to be accepted on the block chain. That incurs a relatively high flat fee (somewhere between $5-$50 dollars in recent times). It doesn't make sense to pay this fee for small amounts (though is great for large amounts). You can get around this by using exchanges and the equivalent of credit cards where multiple transactions are rolled up into one wallet to wallet transaction. This comes at the expense of volatility or if your bullish it comes at the expense of having to float a growing bitcoin reserve. This is not an option for a lot of businesses, basically you need a 3rd party provider for it to make sense for these businesses and realistically that means they're getting cash value at time of trade and it's the provider that is taking on the volatility risk/reward of waiting to trade there bitcoin.