Dating takes time, weddings cost money, and its hard to drag partner/spouse/family around the globe for each new short-term position. I once worked in a lab that joked about a “celibate for science” lifestyle. I think the PI really actually wanted that for his grad students.
My sister and BIL are both academics, and didn't "live together" for probably around 10 years, 6+ of which were after marriage. They bought a house together in the city my sister taught in, and my BIL commuted on the weekends sometimes, and they spent summers together, but it took the better part of a decade for them to both land jobs in the same city. It's incredibly common.
The logistical stuff is difficult as a postdoc. A traditional wedding is fairly expensive and time-consuming to plan, and many postdocs are cash and time poor. People often like to have the ceremony near family—-isn’t the bride’s hometown traditional?—-but people move, often very far, to find good positions. Traveling back and forth is also tricky if you have experimental time points or lab animals to take care of, as we did.
More psychologically, being a postdoc is a weird transitional state. I felt odd promising life-long commitment and support to one person, while not knowing if my own job was still going to support me next year. (I was on one-year renewable contracts).
Finally, we were a little concerned that being married could make things more difficult for my wife. Although they shouldn’t, a surprising number of profs are reluctant to take on a postdoc who might be ‘distracted’ with a spouse or kids. Furthermore, if you have a kid as a professor, you often get an extension in the tenure clock, help with daycare, etc. As a postdoc, you can maybe get a little more time on these time-limited fellowships, but that’s about it.
We eventually did get married though (and it’s been great) but waiting was not totally irrational.
