This site sucks, I found a NIH study from 2002 on the subject
https://pubmed.ncbi.nlm.nih.gov/12369203/
EDIT: OK Wikipedia to the rescue: https://en.m.wikipedia.org/wiki/Caries_vaccine#:~:text=mutans.,%2C%20require%20occasional%20re%2Dapplications.
"On a different line of research, Jeffrey Hillman from the University of Florida[6] developed a genetically modified strain of Streptococcus mutans called BCS3-L1, that is incapable of producing lactic acid – the acid that dissolves tooth enamel – and aggressively replaces native flora. In laboratory tests, rats who were given BCS3-L1 were conferred with a lifetime of protection against S. mutans.[7] BCS3-L1 colonizes the mouth and produces a small amount of a lantibiotic, called MU1140,[7] which allows it to out-compete S. mutans.[8] Hillman suggested that treatment with BCS3-L1 in humans could also provide a lifetime of protection, or, at worst, require occasional re-applications. He stated that the treatment would be available in dentists' offices and "will probably cost less than $100."[9] The product was being developed by Oragenics, but was shelved in 2014, citing regulatory concerns and patent issues.[10] In 2016, Oragenics received a 17-year patent for the product.[11]
On rare occasions the native S. mutans strain escapes into the blood, potentially causing dangerous heart infections. It is unclear how likely BCS3-L1 is to do the same.[12]"
TLDR; Sounds like the risk of it entering the bloodstream and causing heart problems is what stopped it from advancing.
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