i'd personally say it's down to a few factors:
as has been touched on earlier, most of us are getting on- as are our cars.
the number of 5's has rapidly decreased in the last 10years- we are now a smallish club, rather than a medium sized club.
the cost of fuel has really put a dampener on just travelling across most of the country to meet up with a few mates for an evening. - it does go on, but it certainly used to be alot more widespread.
if people think that an event isnt going to be well attended, then it becomes a self fulfilling prophecy, as people can't be bothered to travel to stand on their own somewhere.
events seem to need a 'draw' - back in the day at the Pod it was GT Tunings car, then it was Ktec's car, then glen tuffnell, then DD autosports, then Mike Spencer. every time people knew they were going they would turn up. - now its Scoff or Glenn pulling the crowds, be it at a rolling road day or drag strip. - i can guarantee scoff's RR day will be very well attended.
as has been mentioned before, the goalposts have been moved along way from what the average gtt owner can afford, so power tussles dont seem to occur now. - which was another reason for people to go to 'competitive' events.
i'm sure i could come up with other reasons off the top of my head, but i'll clear off back into the 'website editor' corner.