That's the main issue, I think.  There's no way in hell I'd want my pension funds invested in something like bitcoin, especially if I was close to retirement.  It's way, way too risky.

There was a time not too long ago when the market cap of bitcoin was probably too small for big money to invest in it.  I just checked bitcoin's market cap, and it's $108 billion.  That's definitely not too small for Wall Street to get involved.  The market cap of Dominos Pizza is just over $10 billion, and a lot of mutual funds have DPZ stock.  The big players probalby don't want a huge chunk of volatility and uncertainty in their portfolios, that's my guess. 

On the other hand, there was just a thread in Economics about how some big players were buying coins over the counter, bypassing exchanges--so there are some investors with deep pockets getting into crypto.  It just hasn't become widespread yet.