To explain it somewhat, that was also pre-MAD (in the modern sense, since ~1975), which meant a combination of (a) sudden potential existential threats & (b) ra

To explain it somewhat, that was also pre-MAD (in the modern sense, since ~1975), which meant a combination of (a) sudden potential existential threats & (b) rapid technological change. In that environment, it was more reasonable to run parallel development projects, as one didn't want to be caught flat-footed if one approach failed (e.g. SLBM + ICBM + bombers).