It's a number that was pulled out of the air by the New Zealand version of the Fed in the late 80s, and it seems to work well, so everyone copied it.

I think it's worth pointing out that it's all part of the neoliberalism fad that originated around that time. There are other ways to run an economy, but this inflation target one works well for neoliberalism because the wealthy own assets which hold their value through inflation. The middle class and below have money, which loses value through inflation. They pay what money they do have largely (directly or eventually) to the wealthy who then use it to buy more assets.

Sorry I may have channelled too much Gary Stevenson there.