toto was wrong. very wrong...
so, all the teams have different dynamics in terms of their relationship with vampirism. some teams have strict policies and ask that all staff become vampires. others might ask for just major players (sporting directors, engineers, etc. etc.). rare cases might just be do whatever you want free reign humans and vampires vibing how they please. mercedes acts like a more free reign environment with vampires and humans kind of meshing together. [but also a lot of people who have been there for a while are vampires (shov, bono, ron, simon etc. etc.). which is why toto probably thought at some point james would change his mind abt it]
anyway when james turns into a vampire toto thinks he's going to stay; not a big deal that he's changed, he can move past this, and that this is just a minor obstacle.
except when james gets insane bloodlust, goes fucking crazy [rare occurrence that can happen when some ppl turn], and starts killing people left and right. at first everyone thinks it's just typical beginner vampire stuff then they realize he is just so insatiable that working with humans is just triggering his hunger all the time.
toto tries to find a solution because he doesn't want to lose james but jost steps in and convinces james to leave for williams because williams has a strict policy of everyone being vampires there. [not important but jost knows something really bad is about to happen in the 2023 season and uses the opportunity to jump ship]
so yeah. long answer. basically he leaves because the merc work environment was not working.