Only because they were on the BBC with a captive audience, not because of huge interest in RL.
And that actually makes it worse. Not only was there P&R but famous players and lots of coverage (allegedly) So why weren't RL club crowds as big as they are now? (When RL is apparently at deaths door -again)
I think you'll find that their are less people watching RL now than 20 years ago.
At that time teams like Oldham, Halifax, Featherstone, Leigh, to name but a few could regularly attract 5,000+ crowds. Now they would be lucky to get 5,000 between them. All that has happened is similar/less numbers of people are watching but are spread around fewer medium sized clubs.
SL has led to the demise at least in part to many clubs who had a decent following...This is, after all, exactly what the RL want. The only way to get close to the Australian model is to have 1 (super)league and b*gg*r everyone else.