Competing in Championships!

This has always Puzzled me now I wonder if any Drivers out there can help!
With the Two Current Rallycross Championships we having running at the moment why is it that particular drivers in all categories stick to one Championship and not partake in the other.Surely competing on all the different circuits and the Extra valuable track time in the cars can only help improve the drivers and help them be more competitive,Now I know expense is probably the main cause but there are drivers out there who I'm sure could probably afford this.I would certainly want to do as many events as I could if I was a driver, and certainly do my national championship as well as other events.
Are there incentives to stick with one championship or are some off the drivers afraid of upsetting the organisers by competing in both Championships! It would only better for Rallycross if we could get as many cars at events as possible who knows we could maybe get back to the days of C,B and A finals and even full grids for the Div 1 spec cars now that would make the Europeans sit up and take notice!

As you say cash may well be the determining factor in a lot of cases. Other than that some cars are not eligible for both championships, others may be uncompetitive.

i guess also if you're chasing a title in one championship you may not want to risk the car competing in an event that doesn't give anything towwrds your title attempt.

If that's the reason why in britain competitors choose one of both championships, then why drive rallycrossers in the benelux and the belgian and the dutch championship, and are there some who even add some english or german events?