[1] suggests that it's a combination of it being at least mildly hard, and there being insufficient fiscal incentive to develop it.
It seems several vendors have realized solutions in varying stages of human trials, without getting to the "commercial" stage (yet):
- "CaroRx" is entirely not mentioned on the owning company's products list [2]
- The list of authors on the antibiotic study from the Chinese Academy of Sciences don't seem to have published any other studies on this since the 2013 one [3]
- Oragenics seems to have just gotten a patent on a new form of their one-off replacement bacteria [4], but that's not nearly the same thing as human trial approval
- The US military's gum for mitigating issues with dental neglect that happen in combat seems to currently be attempting to get licensed to non-military vendors for civilian use. [5] [6]
[1] - https://en.wikipedia.org/wiki/Caries_vaccine
[2] - www.planetbiotechnology.com/products.html
[3] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3708248/
[4] - http://www.oragenics.com/news-media/press-releases/detail/35...
[5] - http://www.newyorker.com/tech/elements/zero-dark-cavity
[6] - http://techlinkcenter.org/summaries/antimicrobial-peptide-an...