You are a saboteur trying to blow up nazi assets in occupied territory. The only equipment you're able to bring to the site of detonation is stuff that will be unobtrusive under your street clothes, since the enemy and collaborators are everywhere. Wire itself is considered war materiel and not readily available.

Your question kind of answers itself - if they could run the wire all the way between detonator and blasting cap then they wouldn't have to splice it.