A major reason was that there was a dogma that language is uniquely human phenomenon. It was already part of the hugely Decartes' philosophy of mind and got a strong revival with Chomskian linguistics and the "cognitive revolution" in the late 1950's.
There was sporadaric research into animal linguistics, e.g. the Koko study, but those were dismissed on the grounds that animals can't have language by definition.
The ethics of enslaving and torturing animals is definitely part of the motivation for the dogma.
There was sporadaric research into animal linguistics, e.g. the Koko study, but those were dismissed on the grounds that animals can't have language by definition.
The ethics of enslaving and torturing animals is definitely part of the motivation for the dogma.