There are many reasons, mostly political. One of them is the stance on licensing, another one is similar to how many people avoid using Blink-based web browsers, to avoid GNU dominance on UNIX.
Two example are POSIX sh, where many people cannot tell difference between POSIX sh and GNU bash, or GNU coreutils (and other GNU utilities e.g. GNU tar) which contains many non-standard flags that are not present in POSIX.
Two example are POSIX sh, where many people cannot tell difference between POSIX sh and GNU bash, or GNU coreutils (and other GNU utilities e.g. GNU tar) which contains many non-standard flags that are not present in POSIX.