One of the issues is that their historical strengths became weaknesses. Scouting integrated into existing infrastructure, which is why religion is such a prominent aspect. (It’s also why as mentioned elsewhere the politics around the LDS church became so recently important.) Boy Scouts was mostly and overlay that slotted on top of church youth programs. (Also other secular groups but that was smaller)

The shifting of religious practice in the US impacted scouting as well. Mainline Protestantism and Catholic Churches are on the decline - that’s the backbone. In the Catholic environment I grew up in, Boy Scouts kept kids engaged after communion with the parish.

The other issue with the model is that the local organization leadership reflected the old model. (ie. It’s a bunch of white dudes) The most traditional, growing communities who would be attracted to scouting with Catholic and Episcopal communities are Hispanic, Filipino and in my area Indian.

It is sad. I was involved from age 7-14 (when we moved) and loved it. But institutions only survive when they can grow themselves.