Presumably, because it'd be annoying waiting for lame duck mode when you actually do want the application to terminate quickly. SIGKILL usually needs special privileges/root and doesn't give the application any time to clean-up/flush/etc. The other workaround I've seen is having the application clean-up immediately upon a second signal, which I reckon could also work, but either solution seems reasonable.
Using SIGTERM is a problem because it conflicts with other behavior.
For instance, if you use SIGTERM for this then you have a potential for the app quitting during the preStop, which will be detected as a crash by Kube and so restart your app.
