Why is dyeing hair not obligatory?

The prophet صلى الله عليه وسلم said: > “The Jews and the Christians do not dye (their hair) so do the opposite to them” as recorded by Al-Bukhaari, Muslim, Ahmad, Abu Daawood and An-Nasaa’i. Yet no scholar ever stated that dying hair was obligatory. Why was it so? Any command from prophet (saw) is obligatory unless evidences suggest otherwise. So were there any evidence as to why dying hair is not obligatory, can you please quote them?