The cost of the Ni-Fe units was more than double, but the main reason for ending their fitment to motorcycles seems to have been that the scarce materials were needed elsewhere. I believe that there were several aircraft applications. A magneto-equipped motorcycle with a flat battery is not the end of the world. Sulphuric acid swilling around inside the aluminium structure of an aeroplane is probably not a very good thing. The Air Ministry had a talent for getting what they wanted, even in peacetime.
It seems that one of the problems with the Nife battery was its poor performance in cold weather, the winter of 1939/40 was very cold so maybe that had something to do with the rapid change to Lead Acid batteries early in the war?
Had the 1.2 volt figured out but the only batteries I have ever delt with over 50 odd years have all been 2volts per cell, excluding wireless accumulators , you live and learn