The 1950s were the days of the Hays Code, which dictated everything a movie could show us. The code was mostly prohibitive of sexual themes and images; violence just had to be handled with "special care," i.e., not too graphic or gruesome. This kept Rear Window more or less out of trouble.
At the same time, however, we're talking about a murder here. And it's a messy one, with Jeff and his friends speculating about bodies dismembered in bathtubs and the splattery messes that ensue. We never see that gore, of course, but Hitchcock is happy to let us contemplate the gruesome details.
He also gives us a few pretty steamy scenes, where Lisa is doing everything she can to get Jeff's attention without violating the Hays Code. That, plus the generally adult theme of voyeurism, makes us think that a PG-13 rating is about right. But honestly, if it hadn't been for that strangled little dog, we might have said PG.