The same reason they trialled dropping one out if the back of a C-5 - airborne alert.
A problem of the silo based Mx is that at of a response to an enemy 1st Strike - If a threat is inbound you can either use it or lose it, losing the option to let it play out out before responding. With an airborne response you can afford to wait till after the strike to assess and respond.
But isn't that the role of the SSBN force, you may ask.
Firstly the B-70 was in the era before Poseidon/Trident, secondly it would have provided redundancy, and a raison d’etre for SAC (which it subsequently lost).
Last edited by ORAC; 25th Nov 2020 at 14:20.