Most single-issue parties fail precisely because they don't want to broker power, they just want to get the one thing they're angry about out of the way. Most people are expecting the party they vote for to have an answer on every issue imaginable, and merely shrugging your shoulders and saying "I was elected to legalize copyright infringement, not to decide what our external immigration policy should be" is not an option. If you don't play power broker, you do not get your agenda done.