There are additional capital requirements for brokering stocks, as mandated by regulations including Dodd-Frank, that made it extremely expensive to handle GME trades during the frenzy.
This thread explains the math in detail: https://twitter.com/kralctrebor/status/1354952686165225478?s...
I know the popular narrative is that this was some sort of conspiracy, but the truth is likely far more banal. Unprecedented volatility and volume are exceeding the limits of these systems. No one budgeted for a single stock dominating news headlines and swinging wildly while millions of retail investors tried to enter at the exact same time.