That’s the fascinating thing about Formula 1. Being a good driver is only half of your job. You have to be a good team player, too.
But also you have to be a master of team politics. At the same time as you are trying to do the best for your team, to keep your race seat for the following season, you’re trying to screw over your teammate as much as possible, and trying to get better treatment than your teammate from the team, and trying to get the team to turn against your teammate.
Nico Rosberg tells a great story about his time at Mercedes as the teammate of Michael Schumacher. Schumacher did everything possible to put Rosberg in his place, and was utterly ruthless. In this case, there was only one toilet in the team garage, and the drivers would always have priority due to their schedule being so set during the race weekend. Nico knocked on the door and said “hurry up, I’ve got to start qualifying” but he didn’t know it was Schumacher in the toilet. Schumacher stayed in there for another six minutes, making Rosberg wait outside. Then Schumacher left the toilet, not even making eye contact with Rosberg.
Turns out Rosberg wasn’t naturally that scheming and political, and had to make a real effort to practice it and try to screw over his team mate. He couldn’t keep it up, so retired on winning his first championship.
So when he left Mercedes, after their experience with Schumacher vs Rosberg and Rosberg vs Hamilton, the team went for the least-political driver they could, one who would fit the car’s racing handling like Rosberg but come with none of the drama.
Also Bottas happened to be managed by Toto Wolff, the team owner. Which is kinda handy for him.