I think SCCA missed the boat years ago. Class consolidation and making ALL classes that make the numbers Runoffs eligible. But, that threatened pet classes of the gate keepers, sooooo, it was deemed Such a move would ruin what is SCCA only 'entry level racing category'.
Prod was drying up, but, they woke up and smelled the coffee, and drafted new rules that tempted many IT guys who wanted to be Runoffs bound. Then, the ST category was created by those who wanted IT to be a Runoffs eligible category, and poof, more defectors.
I haven't looked at subscription numbers, but my general gut feeling is that IT is 33% double dipping Miatas of one form or another. Not everyone wants to focus on the Runoffs though and certain regions have great fields and racing. Which is cool, but I think the club missed a great opportunity to increase participation, rather than just move participants...