I think a large part is due to the additional regulations that have been placed on the private sector that is inhibiting growth. (And before someone jumps in, as they inevitably will, and claim I want zero regulations, that is not what I'm saying and please don't come with that straw man.)
Entrepreneurship, new businesses, is actually down in this country and that's where a majority of our growth comes from. Small banks are getting punished by Frank-Dodd and as such are lending less. Small businesses get most of their loans from smaller banks so you where this is going. Between the regulations Sarbenes-Oxley and Frank-Dodd have put on the private sector it's not surprising we are seeing less companies go public, especially these unicorns.
America's private sector is the greatest growth engine in the world. There is a definite need for government here as they have to set the rules and enforce them but it's not gov't that's creating the jobs and leading economic growth. We need to unleash our greatest economic weapon, not suffocate it as we are now.