for many reasons I know nothing of

At that date (1955 model year), Chrysler had never built a "wedge" engine, although they were already on the market for 6 years (Olds, Caddy). It seems obvious to us, but at the time it was a "what if?" concept (prior to that, vertical OHV valves were common), which Buick got wrong at first.
After a few years, it looked like the wedge would work (the "B") and the valve gear would be cheaper than the poly. Finally, the LA in 1964.

In summation: cost and weight.