From what I know, it seems good, so why does no one use butane as a rocket fuel?
Hydrocarbons are largely interchangeable when it comes to performance, and as long as no single one beats kerosene with a large enough margin, the hassle of changing infrastructure is generally not worth it.
Compared to RP-1, butane has slightly (very slightly) higher specific impulse, and somewhat lower density, largely cancelling each other out.
Butane comes up short, since if sacrificing some density for Isp is beneficial, why not go all the way with methane? And if sacrificing density is unacceptable, more highly refined compounds like syntin gives you the same Isp boost, with even a little higher density as an added bonus.