A while back Harmonix and Rock BandTM ran a crowdfunding campaign to bring Rock Band 4 to PC. However, the campaign has been revealed to be unsuccessful and as such, the following decision has been announced on the official page:
But at the same time we learned exactly what we needed to learn: there doesn’t seem to be enough of an audience to make Rock Band for PC a viable project for us right now. We’re committed to supporting and improving RB4 on consoles. To be clear, we raised nearly $800,000 via backers and investors; it’s an impressive showing of support from our community and for our brand. But as an independent developer we have to be careful about how much money and development time we risk on a project we’re not sure has a big enough audience, and crowdfunding allowed us to (among other things)* judge the market fit for Rock Band PC.
The minimum asking price for the campaign was $1.5 million and it raised $792,817 by the end, a little more than half of the goal.