You don't have to be Mormon or even religious to think that there's value in having a youth program that is specifically male-only, such that you'd be highly motivated to abandon a youth program if it stopped being male-only.

A lot of Scout groups are church based and not only with the LDS.

I have heard that the LDS church got wind of abuse claims within scouting before they hit the mainstream. They have their own abuse scandals just now so probably didn't want to fight that war on two fronts. Either that or cost cutting, which is a major feature of modern Mormonism, except where temple construction is concerned.

Scouting hasn't taken off in Mormon churches much outside the USA by the way. Not really in the UK.