As Bob said, HFT has had an effect and he and I spelled it out a few years ago in ET. But other things are in play as well. I also agree with the cycle comment. It will return one day, assuming their is a wall street or something similar.
Wall street has always fleeced the middle class. They are disappearing and have less money: less lambs equals less wool. The smells from the collusion, corruption, and fraud being shown by some Wall street "traders" is coming home to roost. Most retail now think it is a fool's game and don't want to lose their money. Corporations are buying their own stock and pumping up the market with the help of the FED, but very few are fooled by that ploy.
Inequalities of information flow and inequities in legal charges and insider trading etc. are getting harder to foist on smarter traders who learn faster what to do and what not to do. They don't want to lose to feed the big fish anymore.
My analogy from the past is like strip mining the sea. Each fish needs a promise of potentially making some money or thinking they have some advantage. If you kill all the minnows, then the fingerlings begin to die, and then the bigger fish, and then etc. That is the phase we are in I suspect. Finally when there are no more fish, then the regulators and the industry will suddenly say - hey we need to do something. Then of course it is too late.
There is a simple demographics aspect as well I suspect. Old can no longer afford to fund the big guys. Youth have no money.
It used to be a few big traders and millions of fingerlings. Now is it a few fingerlings and mostly big traders. So volume drops if they don't invest in growth anymore but try to game the other big traders out of a few pennies. One happens to one, happens to us all.
I would love to see, the percentage of people actually holding stocks for a month or more compared to 40 years ago, when people believed in government, the middle class dream (Carlin says you have to be asleep to believe it), and the future. Something changed in 1980 or earlier.