A big part of the reason for that is an awful lot of those within rallying don't understand/see the benefit in spending on promotion.
I've customers in both sectors, circuit racing totally gets the whole image / fan engagement / speculate to accumulate angle. Bar a handful in rallying there's an inward looking attitude and any cost/time commitment outside of the actual car there's total disinterest.
A good example is look where start venues for rallies are and the times thereof