My paper demonstrating a side channel attack on RSA via hyperthreading was rejected from the crypto preprint archive on the basis that it was "not cryptography".

(Reviewers at J.Crypto subsequently sat on it for a year and then suggested I submit it to a journal on CPU microarchitecture instead.)

Novel research is uniquely susceptible to "cool but it's not part of our field", because that critique is entirely correct until the research gets published!