It's seen as too risky because the fluidity of the liquidity of prices within the market. The price could jump high, or fall low, all within the course of a day. That is a huge financial risk for a bank to take part in. Also, the regulations on crypto aren't entirely certain in every aspect.
I will come back to Bitcoin after 5 years. If Bitcoin is there, it will go the silver way. Just as Silver was $80/ounce in 1980, Bitcoin can go back to $200.