Why the M50 replaced the M40

Possible reasons why the Avon M50 replaced the M40 protective mask in US army service.
The M40 had a few known problems, it was made from silicone rubber not butyl making it vulnerable to blister agents and other chemicals. The C2 filters it was issued with had a leaking chromium problem which caused a small scandal and investigation over their safety and the US wanted a mask that was better at protecting the user during filter replacement.

Personally I think the C50 in 40mm would have been a better choice than the M50.