Because it is probably BS. I doubt you can sterilize someone with a vaccine.
Well here is the National Institute of Health saying it can be done back in 1993:
http://www.ncbi.nlm.nih.gov/pubmed/7683889
Population Research Institute from 1996:
http://pop.org/content/bad-blood-in-the-philippines-1200
They have a patent:
http://www.google.com/patents/US5733553
The European Society for Human Reproduction and Embryology from 1997:
http://humupd.oxfordjournals.org/content/3/4/301.full.pdf