1. They had competent PhD advisors 2. The advisors had stable funding sources 3. They were single
Of those three, #1 and #2 were by far the most important. Certain professors just knew how to run a good lab and were able to shepherd their students through the program efficiently.
As for the impact of #3, I found as a married student I had to balance my research and teaching responsibilities with the needs of my spouse. It added a level of mental and emotional stress my single colleagues didn’t have to deal with.
Ultimately, my balancing act was unsuccessful. I eventually dropped out of my PhD program and ended up divorced.
So yeah, based on my anecdotal (N=1) experience, being married doesn’t not help you to be successful in graduate school.