They CAN fire at long range, but it's incredibly inaccurate. Even most contemporary ships aren't all that accurate at long range, however. And the power levels are roughly equivalent by that description to, probably, a high-end nuclear plant of today. Plus, look at the piloting. Rebels are inventive, cunning, and (in some cases) outright insane. The Imperial forces are drilled, formation-oriented forces. It's redcoats vs colonials.