Steels with high carbon equivalent have poor Weldability, because in these steels during welding

Carbon and other alloying elements get oxidised from the weld pool

Excessive ferrite forms in the heat affected zone leading to poor toughness of the weld

Martensite forms in the heat affected zone leading to poor toughness$/$ductility of the weld

Segregation of carbon and other element occurs in the weld pool leading to poor properties of the weld