It's a combination of things.
Super League started and got weekly coverage. 20 years ago, only the cup competitions were televised, so they were special.
Then the Grand Final was introduced, which I love, but given that winning the GF is the culmination of a whole season's hard work, puts that ahead in terms of it being an achievement. To win the Challenge Cup, for instance, you only have to win five or six games, against the 20-odd it takes to win Super League.
Moving the whole sport to Summer didn't help the Challenge Cup either. For several years it was effectively a pre-season tournament, as opposed to something which took place to pretty much cap the end of the season.
Then, Wembley closed. The CC was about Wembley - it's as simple as that. Yes, we have the new Wembley, and it's technically better, but it's not the old Wembley, is it? Be honest - winning the CC at Murrayfield was great, but not as great as if we'd have won it at the old Wembley, is it?
In a nutshell, the whole sport has changed, but the CC hasn't, really. I rather fancy that only a damn good marketing campaign, sustained over several YEARS with Wembley as the focal point of it, along with it remaining on free-to-air TV, will stop the decline.