For two reasons mainly, because it has been used to treat other protozoans such as Leishmania and Naegleria fowleri and because its chemical structure closely resembles rifaximin. It's lipophilic so it should have no problem getting into the protozoans.
Whether its effective is another matter. It has many side effect and it's hepatoxic, not on my list that's for sure. Rifaximin is a better choice because it's poorly absorbed from the gut unlike rifampicin.
Generally there are many antibacterial drugs that has an effect on protozoans. They have much in common so it's no surprise.
Btw you are in Germany right? Did you try the triple cocktail?