Well. With all due respect, if you want to keep racing in regionals, you are free to do so under either arrangement. Why then would you desire to keep those that would like to run nationals from doing so?
Those who would like to run nationals already are free to do so. They just cannot do it in an a class that was not intended to be subject to the entire cluster fornication that accompanies a non-spec, non-sealed, national class.
Those that would like to run nationals already are free to do so. They just cannot do it in this category; a category that was not intended to be subject to the massive cost inflation associated with being a national category.
Those that have newer cars and would like to run nationals are free to do so. They simply must run their cars as BP and DP.
The only reason I can think of is being afraid of a shrinking grid.
Cost: The relative cost of building, prepping and maintaining an IT category car is going to go up dramatically. The distribution of the cost bell curve will shift to the expensive end - and that's just to maintain your relative position.
Time constraints: DC Region's National typically has 225+ cars over 8 groups. Where are we going to fit the extra cars? Run ITC in small bore with an 11+second/lap difference between ITC and EP and a 9+ second difference between B and EP? That's more than the ITC/ITS gap and we've already realized that it doesn't work for a decently subscribed sprint race group. remember, National track sessions are longer and must be adhered to. No shorting sessions because the day is running long.
Dander: Topeka wants to suckle at the teat of the ARRC and ITFest. Having treated IT like a bastard step-child, unfit to associate with the gentry, I see no reason to recognize Topeka's attempt to invite us in to sit with civilized society since the only reason they are doing it is because they see us as a cash cow. Having completely fubared the Runoffs (tm) by allowing so many National classes to wither and then putting the patient into shock through Startline Steve's ramming Heartland Park down the club's throat, Topeka wants one thing and one thing only - our money. As it now stands, IT has its own "championships" and Topeka has got its panties in a knot because they aren't getting their blood money.
And, yes, moving IT to National will dilute car counts. Budgets are limited. If I've got enough money to run 6 weekends, then increasing my options doesn't add more money to that pot.