Jon, can you count how many NHRA caliber funny cars there are in the country? Geographically the independent teams are based well outside a reasonable range of Indy (i.e., CA/FL) while others may be "NHRA legal", but haven't figured out a combination. With the cost of fuel racing today, its not worth it for most independents. to make the trek or take the chance. The independent teams that do not have these issues are entered for Indy.
Top Fuel, I would argue differently. Most of the independent fuel teams are based within a reasonable town to Indy and from past races, seem to have a combo that could get them qualified. Again, cost plays a huge role.
I am stunned there are 25 fuelers at the Big Go....but maybe we are healthier than we think