The main reason, as far as I'm aware, is because McCulloch then went on to claim to invent several thrusters (EmDrive, most notably) derived from the effect that break normal conservation of momentum.
In what sense does McCulloch claim to have invented the EmDrive? The only citation about him on the EmDrive wikipedia article isn't even talking about anything relevant to the purported mechanism of action of the drive.
The theory behind EmDrive is based on his quantized inertia theory. I was under the impression he was the original proposer of the idea. It looks like it was instead proposed by someone else in 2006; however McCulloch submitted a paper in 2013 supporting it (https://arxiv.org/abs/1302.2775) and publically advertises about how his theory allows for "an electric rocket" on his twitter https://twitter.com/memcculloch