It's on Steam Greenlight... You have to vote to get it put on Steam... so go vote!
"Many of you have been patiently (OK, not so patiently) awaiting the release of the PC version of the Pinball Arcade. We are excited to announce that it is finally ready! We would like to partner with the Steam online gaming platform to distribute the PC version, but we need your help. Steam uses a greenlight process to decide which games they distribute based on the number of votes a game receives. Please take a minute to cast a vote for the Pinball Arcade. As a small thank-you we've put together a new video that includes a glimpse of the upcoming Elvira and the Party Monsters table- check it out and cast a vote by following the link below!"