In low voter turnout local elections driving out Republican voters is more important than trying to persuade crossover. If you get 100 more Republicans to vote in a local partisan general election about 98% are going to vote for all your candidates. Compare that to trying to appeal to 100 Democrats to crossover. You may get single digit % of Democrats to crossover. 98 more votes from Republicans is better than say even a generous estimate of 9 Democrats crossover. Also, easier to turnout Republicans than get crossover.