When the F-105 was first designed, it was, pure and simple, TAC's purpose-built nuclear bomber. Starved for funds by the then more glamorous SAC, TAC needed to compete.
Unlike the F-100 and others pressed into nuke deliveries, the -105's fast and low design, as well as its cavernous internal bomb bay (Buccaneer anyone?!), was optimized for the tactical nuclear role.