They need 6 days for the rugby tournaments, so that slots in nicely at Stade de France before the athletics competition begins, especially since unlike pretty much any other Olympics, they're not using the stadium for the opening ceremony. The football tournament is spread out over the length of the Olympics, so they put that at Parc des Princes. I can't speak to French rugby, but I know they had the World Cup at Stade de France, so if they can get the big crowds there for the Olympics, it makes a lot of sense to have rugby there