UK Nature and Environment

Two types of tuberculin are injected into the cow's neck; one bovine and one avian (the control). Three days later the resulting swellings are compared. A large enough swelling of the bovine injection site compared to the avian one is interpreted as evidence of an immune response having been mounted against it because the cow has previously been exposed to the bovine strain. In a cow that's been vaccinated, the skin test provokes an immune response in the same way as in a cow who is actively infected, rendering the test useless for detecting active infections i.e it produces false positives. Nobody will import your animals if they can't tell whether they've been vaccinated or are ridden with an infectious disease. A couple of countries have chosen to simply sacrifice exports and get on with vaccination, which is good in terms of preventing tens of thousands of cows being needlessly slaughtered early but it also means they can continue to keep the cows in the conditions that make diseases like TB spread in their herds. It's bad for the animals quality of life but it's also stupid because TB isn't the only disease which will emerge in herds kept in those conditions.

TLDR it's all short-term profit-driven thinking.