Related question: Why is welding pretty mainstream while blacksmithing is a much more niche craft? Blacksmithing is a more overarching skill: After all every blacksmith knows how to weld but relatively few welders can forge effectively.
Possible answers:
1. Blacksmiths enjoy making custom tools for each domain while welders just want to get on with solving their domain problem.
2. Blacksmithing is harder to learn. Welding using modern techniques is easy to learn. (Caveat: Welding well is quite difficult. But learning to weld good enough to repair a broken hitch on your tractor is easy.)
3. Welding can solve a very large chunk of metalwork problems. Not all of them--and not always with elegance--but it gets the job done quickly. Blacksmithing can solve a larger set of metalwork problems with more elegance but it also takes more time and skill.
Possible answers:
1. Blacksmiths enjoy making custom tools for each domain while welders just want to get on with solving their domain problem.
2. Blacksmithing is harder to learn. Welding using modern techniques is easy to learn. (Caveat: Welding well is quite difficult. But learning to weld good enough to repair a broken hitch on your tractor is easy.)
3. Welding can solve a very large chunk of metalwork problems. Not all of them--and not always with elegance--but it gets the job done quickly. Blacksmithing can solve a larger set of metalwork problems with more elegance but it also takes more time and skill.