Why did scholars consider shaving the beard to be unlawful, even though the Prophetic Hadith, “Let the beards grow and trim the moustaches,” is a command to let them grow and not an explicit prohibition against shaving them, while other command-based Hadiths, such as “Take the pre-dawn meal, for in the pre-dawn meal there is blessing,” are not considered unlawful to abandon?

The default rule for commands from Allah and His Messenger is obligation (wujub), and for prohibitions, it is prohibition (tahrim). However, a command may shift to being recommended (istihbab) or permissible (ibahah), and a prohibition to being disliked (karahah), if there is evidence or an indication. The command to grow the beard (i'fa' al-lihyah) remains obligatory, thus shaving it is forbidden. As for the command to have suhoor, various evidences and indications have shifted it to being recommended (mustahabb), including the Prophet (peace be upon him) and his companions continuing their fast without suhoor (wisal), and the consensus of the jurists that it is recommended and not obligatory.