Two thoughts:

1. The 70s-80s were "power mad". I was a teenager then. We all wanted "full hunting legal limit 12 ft/lbs power" even if that meant we ended up at 14 trying to manage an HW80, and probably failing.

2. The Original Dianas were better marketed than later (or now) but they were typically anything from the same price as a BSA Meteor (24 and Meteor both about £44-45 in 1984) to 5-10% more expensive. Those youth guns were mostly bought by dads - who (a) probably didn't want to spend more than needed to start junior shooting, (b) still had a patriotic attachment to British brands.