Unless about 75% of PA Democrats are sexist, the issue is really that there just was never much excitement about Allyson Schwartz’s campaign.
She never gave the electorate any real reason to be excited about her. She had a pretty blah Third Way voting record in the House, and wasn’t promising voters anything especially compelling compared to the other candidates. That’s why Schwartz peaked in November 2013, way before the race even started for real. And that’s even being generous. If you incorporate her internal polling into this chart, her support peaked in early July.