Driftless
Donor
I can think of a couple of reasons: (1) its performance was known. Basically, "the bird in the hand is worth two in the bush" kind of thought pattern. (2) The more cynical, whoever was peddling the Lanchester said they'd have a thousand units delivered in 10 days - or some other equally bogus timeframe.The RAF did actually have a legitimate need for SMGs - got to remember that this was at the height of the invasion scare and it was expected that German paratroopers would attempt to capture British airbases to pave the way for a land invasion. The Lanchester wasn't just for the Navy, it was for RAF issue too.
The Lanchester was a good piece of kit, but it's just slightly odd that the Ordnance Board would pass up a substantially cheaper and more efficient SMG in favour of an outdated M.P.28,II clone, especially since there was such a panic about mass-producing SMGs as quickly as possible.