No legal access points is the obvious answer. They want clearing houses, custodian accounts, insurance etc - just like with other financial assets.
source https://www.google.com/url?rct=j&sa=t&url=https://www.reddit.com/r/CryptoCurrency/comments/9oavja/why_investors_from_wall_street_do_not_run_in/&ct=ga&cd=CAIyGmM2Nzk3ODA2ZWViNjVjODI6Y29tOmVuOlVT&usg=AFQjCNEYrABGdqFfUjTJspu0aa1697740A
