Because https://blockchain.info/charts/market-price

Because BTC and blockchain utility do not need to correlate to the exchange rate against USD for the system to have massive inherent value.

[Edit: removed redundant 'correlate']

People are spending less in smaller transactions more often, indicating people are actually using btc for exchange of goods and services rather than as an investment. The spikes in dollar volume were when the market was making major movements, and people were moving USD into and out of BTC in huge quantities.

The fact that btc generation slows down over time also means that while transaction volume will go up total monetary exchange won't go up as fast because its limited more by the size of the economy (how much money people have in btc to spend) than in how they are using it.

The number of transactions is probably proportional to something like the absolute value of the first derivative of the change in value. So basically, when the value is fluctuating quickly, there are more trades/transactions.