People make too big of a deal out of trading volume decline. It means nothing.
First of all lets point out Dodd Frank and the Volcker rule leading banks to close down their proprietary trading units across the board. So theres that obvious part of the equation.
The capital and liquidity is still there. The potential for increased trading is there. In fact, hedge funds have been gaining in sophistication with electronic trading, so the potential to ramp up volume is there. Market participants are just staying opportunistic. They come in when see opportunity. Trading volume is not an indicator for market direction or signal of market confidence.