If I had to guess? It's because at one point it took 24 hours to get your results back so they picked that point so you'd have time to get it back before your flight. Then between really stupid people, tests getting faster, and the idea of the test not being a long term guarantee that you don't have Covid being spread more than it's being understood, that deadline just to have the results in time became a strict window during which the test must be taken.