I know, but as I've said in other comments, hacking a big county is much harder. There are hundreds of polling places and machines, thousands of officials, news organizations conducting exit polls and watching live results, and probably good polls conducted just before the election. It's much more appealing to pick a few smaller counties with limited to no polling and easily exploitable systems, so you can remain undetected. If you concentrate your efforts in a few highly populated and highly watched counties, it'll be obvious.