No, it's because banks offer loans based on the value of the collateral (house in this case) and the credit risk of the borrower. A friend sold a highly graded Lebron James rookie card for something like $15,000 last year and the mortgage lenders wouldn't touch it for 30 days because when cash shows up in accounts without a prior history, they want to make sure there's no unposted debt that matches it.
There may be some element of conversion risk--if your banker can't figure out how much your crypto will be worth in 30-60 days when the sale happens, they don't want to underwrite any risky transaction. What if there's a dip and you're just a little short? Bankers like the easy math (not because they're stupid, but they may play dumb on purpose.)
There's also probably some question about where the money comes from--in the US for instance if you generate your cash through illegal activities the Feds can still come in and seize property. This has created an interesting/weird economy in Colorado in recent years because selling marijuana is legal in CO but it's still a US crime. So banks won't house any money for the businesses/owners because they don't want to risk it being seized by the Feds*. People who generate legitimate income through these related businesses will take their cash and buy houses/land/property in a cash sale; now the property can generate income that has no risk of recapture or it can be sold and the money is "laundered."
But the main issue is proving credit worthiness, so cash out your crypto and let it sit on a balance sheet as actual fiat currency for 30-90 days and you'll get around this issue.
Also, on the second comment, just no. Banks don't create currency, they hold currency. They create credit/liquidity. Banks will gladly hold your funds or lend you funds, but are risk averse, and they apply their risk aversion equally to personal credit risk and bureaucratic legal risk. Banks don't really care where your money came from so long as it doesn't put them at risk.
*this may have changed recently, but this was the case even a few years ago