Why is trichomoniasis so common?

Trichomoniasis. Although the prevalence of trichomonas is estimated at 8 million cases/year, that is probably an underestimate. The reasons are that it is not reported nationally, many people have no symptoms ; the tests used to diagnose it have low sensitivity.
Trich so common. Males typically have no symptoms (only about 20% may have a discharge) so they are unaware that they can spread it. And in females up to 40% may be asymptomatic at the time of discharge. So unfortunately you have a sexually transmitted disease with a fairly high asymptomatic rate - so not having symptoms means not have a problem.