Smoking is considered Makruh only because most gatherings of scholars can’t get unanimity on whether it’s haraam or not.
Most scholars lean towards it being haraam, as it is established to be very harmful towards the body, and poisoning yourself is haraam.
However, a minority is reluctant to declare it haraam because nothing that is physically like tobacco is explicitly declarted haraam in the Quran or Sunnah. They feel that in case they get it wrong, they would be sinning by declaring the halal to be haraam, and decide on the side of caution against outright declaring it haraam.
As a compromise position… all scholars agree it to be makruh.