“I have asked people this question on my own, and I’ve been told by some that they don’t know where the [anatomical] landmarks for CPR are due to women having breasts,” Dr. Nicole McAllister, clinical assistant professor of emergency medicine at the Ohio State University Wexner Medical Center, tells Yahoo Life.

Many people who receive CPR training practice on flat-chested mannequins and learn that CPR should be performed across the breastbone and nipple line, she says. “Because people think of doing CPR in terms of a male-form dummy, some of this doesn’t translate well and they don’t feel comfortable doing it in the right spot,” McAllister says.

From the article. There’s also an expert that bring up your reason, and there are some more explanations given (people don’t realise it when a woman has a heart attack, people are afraid of hurting the woman)