I don't think it was because of Earnhart getting killed. They just lost what made them popular. For instance when they used to come to Watkins Glen international (WGI) places around here had events and drivers would show up with a car and sign autographs. WGI used to also have not for profits work a lot of concessions selling loot and they got a cut of what they sold. Also races were at noon or 1 pm.
... Now, most of the short tracks are gone, there all no longer any more local events because they priced themselves out, NASCAR kicked out the NFP's and took over the loot sales and early races are now 3 pm or night. Throw in ridiculous rule changes like stages and a lot of their base said screw it.
... If I watch racing now its usually IRL, open wheel racing on road courses is a great TV watch.