A contributing factor might be that it is the lightest used of the "deep" tube railways after the Waterloo & City line in terms of number of passengers (at least by pre-Covid measures), 55% or thereabouts of the next highest (Piccadilly).
I think it's fleet size that counts - less than half that of the Northern, Central, or Piccadilly Lines. So it was always possible to equip it with cast-offs, whether from those or (with limitations) the two newer lines. While they too have smallish fleets, you wouldn't want to use the old trains on them (it's virtually impossible anyway).
So cast-offs is what the Bakerloo got. My memory of the Bakerloo by the 70s was that it still had ancient (red!) trains after "our" lines, the Central and Piccadilly, had new silver ones. So did the Northern, of course.