Having read a bit more I think the concern is that if Evusheld is overused, then the current Covid variant may become resistant to it and transform to become a new variant version which Evusheld might not work against (like overused antibiotics create resistance).
But yes i am generally cynical too, just feels the Govt has been dragging its heels for too long on rolling out Evusheld so I do tend to mistrust their silence/rationale, especially when some countries like the US have been issuing it for over 4 months now.