Hello,
the last own developed Leica SLR before the R 3 came out, the Leicaflex SL series, was so expensive in production in such small numbers that Leitz had to sell two additional lenses besides the standard lens with every camera body to come out of the red figures. They could simply not afford the costs for developing an own new modern SLR, they were well behind the technical state of the art in those times. So came the cooperation with Minolta for SLR bodies, the CL and CLE and even some lenses (wide angle and zoom). They had neither the technology and know how nor the money to develop own zooms at that time. Whereas other manufacturers had experiences with SLR lenses since some years, Leitz had no need to use retrofocus constructions at their M's, so they had to begin completely from the beginning in the 60ies.