It's always harder to get accepted onto a joint schools course, because you have to impress two different interview panels from different faculties and they're often looking for different things. PPE is slightly better integrated than other joint schools, of which HistPol is notorious (English and History being another).
Unless something has changed, I'm actually fairly sure that PPE isn't the hardest course to get onto by any measure.
Oxford doesn't offer Politics as a standalone subject, so applicants essentially face the decision between PPE and HistPol. Since Cambridge doesn't offer Politics either, but weird hybrid courses, it also doesn't split the Oxbridge pool in the same way as other subjects. This explains why you see so many politicians going through Oxford PPE.
Again, though, it's worth mentioning that almost as many politicians read Law, not PPE. What courses would you expect politicians to study?