Occam's heuristic and I can provide a simpler explanation with a chart. Transactions are rising, but an order-of-magnitude slower than the market cap.

Bitcoin has limited uses for transactions. Thus the more people in, the more transactions. But it is too limited and not scaling at sufficient rate to avoid a ponzi bubble.

People are transacting primarily to get FX in and out BTC, i.e. most transactions are from speculation and gambling (I read SatoshiDice was 57%). At a much lower level, they are doing some anonymity things or spending it just because they think it is neat (tech nerds) or they think it will help support its value as a currency.