What does the weekend even do to keep people from "getting fiat onto an exchange"? I've never understood this. Any wire will have to be sent days in advance anyway.
I know this may be crazy to conceive but have you possibly thought that there are people who both own a lot of BTC and a lot of fiat? I know, crazy right? Who'd think that the two would be in any way correlated.
Only those lacking in IQ leave a large amount of money on an exchange if they don't plan to actively trade with it. I have both fiat and Bitcoin very little of it is on an exchange.
I could move my BTC onto the exchange right now in response to a market change. For fiat the banks are closed until tomorrow in the USA.
This dynamic makes it easier to push the price down on weekends and holidays. If you want to get a sense of how much of this is going on compare Gemini exchange which requires wired funds with Coinbase app which allows you to buy straight from a checking account and is one of the only weekend/holiday fiat on-ramps.
As of this post
BTC Gemini is $4240
BTC Coinbase app is $4523
Almost a $300 spread. Actual spread is over $300 after Coinbase fees.