I think the statistics simply reflect the facts.
The gen3 Integra just came off of an 8 year production run. A high percentage of them are still on the road.
The gen2 was produced for 4 years, but the newest ones are 9 years old. Perhaps half are still on the road, but not that many enthusiasts still drive them.
The gen1 was produced for 3 1/2 years with the newest now 13 years old. A small percentage are still being driven. There are a dwindling number of enthusiasts.
The gen4 has only been produced for one year. It has a new name, and there are other enthusiast sites dedicated to that particular model.
I see the potential for TI growth to be mainly in gen3 membership due to the sheer number of affordable cars out there. Gen4 membership will grow slowly due to the cost of entry and alternative sites.