The main reason you still find old-school early 80s porn tapes is ironically the VRA which outlawed all XXX on tape in 1984, all of a sudden these pervy tapes became rare so I guess people just shoved em in the cupboard instead of on the shelf
even in 2004 when I worked at Oxfam we would get bags of household bits handed in including loads of old porn tapes, too many copies of Deep Throat to count in fact
you could tell by the dust and musty odor they been there a while, (compared to porn I found very few pre-cert tapes over the course of 9 years)