@Ciello One big reason is that Shin is lacking ambition and doesn't actually put his influence and power to much work in the politics. People who have never met him might judge him based on his fame, but those who have met him would know he's just another teenager who, while deadly against his enemies, doesn't possess any strong opinions, isn't bending others to his will, doesn't boss his underlings around, doesn't scoff at other people's opinions, isn't petty or vengeful, is willing to listen, etc. Any experienced political player who meets him just once will come to the conclusion that despite Shin's awesome magical power, it's easy to influence or even use him. Just like happened with the pope in these chapters. One small bit giving Shin extra character is how he doesn't show super high respect for anyone but is always behaving as if everybody's more or less equal. That's a strength and he can afford it with his personal power.
Marlin and Merida, however, are living legends from many decades ago with lots of important students and friends, it seems, and all in all their way of carrying themselves and looking after their own affairs is different from Shin. Most people would be more wary of messing with older, experienced people than youngsters as well.