They're the most accessable packaging building system, is why. If you're ever actually trying to install something (properly, i.e. not making a mess by just splattering files across the filesystem with 'make install') that isn't already packaged, you're going to have the easiest time with PKGBUILDs (basically, if you can figure out how to compile the software using the instructions for that, you can make a PKGBUILD. The same is not true for other package managers). So they're praised by power users who can manage that and by developers who don't want to learn a harder to use packaging system.