Many people do still want to run FreeBSD, and it may not be you the original developer that has to port it, but the ports maintainer that puts it in the FreeBSD

Many people do still want to run FreeBSD, and it may not be you the original developer that has to port it, but the ports maintainer that puts it in the FreeBSD ports tree, or a company that already has everything running on top of FreeBSD (think Yahoo) wants to use the software, now they have to take the time to port your Linux only code, when using standard POSIX compliant code would have made it much simpler, possibly as simple as a ./configure && make rather than introducing extra work. Why limit your audience to one single operating system?