Because Pleks line plays well no matter who's on it. As long as Pleks is there. So I guess coaches don't bother to dismantle other lines that are working to give players to Pleks. Also, maybe MT thought Bourque's injury was for only one game.
Fine for one game, but if he's concussed I'd move a more offensively minded winger to play with Plek.
While I'm happy to see Ryder play with Eller and Galchenyuk, I think Plek would've been better off with a sniper like Ryder on his wing, which would also mean moving Gionta off his line. I'd be OK with Prust-Plekanec-Ryder, Prust-Plekanec-Gionta not so much.