There are not a lot of differential op-amps around and mostly they target instrumentation amps (so low bandwidth). I'd guess Benchmark couldn't find a difamp they liked for audio circuits. Note the diff-to-se converter is also discrete opamps (in a single package) and not a differential i-amp. But, audio is not my day job, and I have not kept up with what op-amps are available for audio these days.
Years ago I came up with what I thought was a very clever circuit scheme for a wideband differential amplifier. The company thought so too, and I filled out an invention disclosure. A mistake; they were not into patents, just ensuring they had captured prior art in case they wanted it for some reason, and did not pursue the patent. Such is the life of an engineer...