Why do I keep getting trichomonas?

Reinfection. Trichomonas is an std (sexually transmitted disease). Even if you are treated adequately with antibiotics, you can still be reinfected if you have sexual contact with someone who hasn't been treated. That person may or may not know they have the infection. Using condoms will help prevent transmission.
Not certain. But sometimes this is because a sexual partner hasn't been treated. Even if you are treated, you can get reinfected if a partner does not get the treatment as well.
STD. Trichomonas is an std. If you are treated and the infection goes away, then when it returns you have been reinfected. You partner or partners, and all of their partners must be treated to prevent reinfection.