The 30 minute tests are antigen tests. The 72 hour tests are PCR tests. Antigen tests can basically only detect the virus if you're currently contagious. PCR tests can detect the virus before and after you're contagious. Airlines typically require the 72 hour PCR tests.

Here (Sydney Australia) PCR tests are mostly getting results within 24 hours, often as low as 14 hours.

72 hours is a system problem, not a fundamental timeframe for the test.