Probably a multitude of factors, it helps if we don’t start from the presumption of “men driving the change” since the late 2000’s

Technological changes (Tinder, Bumble, Hinge etc) and societal changes with hookup culture changing the “economics” of interpersonal relationships can be contributing factors on top of those two.

We also have lower interest in relationships (long term or fleeting both considered) in general due to economic factors.