First off, the US seemingly didn't start developing a test until after cases were in the US. That is, they failed to use the window they had whole the virus was still contained in China.
Then the initial CDC test was botched and had to be reworked, thus slowing down testing.
Also, apparently the CDC test kits didn't include the needed reagents, so while most large hospitals had stock of those, smaller clinics had to scramble to get necessary stocks, further slowing down testing.
That is, the US-created test was poorly handled and resulted in extensive testing delays.