I may be wrong, but I don't remember seeing markers of this sort when I was younger. I believe their prevalence now reflects the increasing presence of Hispanics in America, and the diffusion of their cultural practices into the mainstream. Here in the Southwest, and particularly New Mexico, these shrines are very common. They come originally come from the Hispanic tradition of the "descanso", which translates as "rest" or "pause". The term is usually used now in reference to a roadside memorial, but its origins were in traditional funeral practices. When a funeral procession was on its way to the cemetery, the bearers would pause for rest alongside the road. Those resting places were regarded as holy ground, and a descanso was erected to mark it as such.