Doctrine and the use of what they had at the time. As discussed upthread it wasn't "because of losses" the airplane was deployed in high sortie rates to some of the greatest risk combat zones.
A lot of the interdiction and strategic bombing during the Vietnam part of the SEA conflict (excluding some of Laos and Cambodia which more often involved close support missions) was a modified version of WWII-style level bombing with radar
Obviously NVA air defenses, AAA, SAMs were a thread in this environment (hence the later Wild Weasel SEAD developments which the F-105 spearheaded and further increased its sortie risk/losses)