@Nicovel0 Ideology, timing, and the structural weakness of the British state basically. The money started to kick in at almost exactly the point thatcher got elected, and a sovereign wealth fund was almost exactly the opposite of her ideology. And she wanted the oil money to help “transform” the British economy, by supporting her plan to break the unions etc.
The Treasury treated oil revenues as income - money to fund current spending - rather than as a capital asset that belonged to future generations as much as to the present. That accounting choice was political, not an economic inevitability.