Maybe the people represented in the Calico Site died out for some reason??
If they were Homo Erectus, I speculate they moved to the Castro Street Site and infected each other with a fatal disease.
Maybe the gap is illusory. Prior to the Clovis discovery (and for some time thereafter, until isotope dating methods became available and accepted), the antiquity of humans in the Americas was 3000 years. The acceptance of Clovis didn't open minds, just dropped the floor beneath which nothing was permitted to fall.
A change of atmosphere will help.