When I was growing up, my father was one of them. Besides a family Bar Mitzvah or something similar, he didn't consider it "going to shul" important to make time for. Many of those "twice a year shul goers," were young. Not only did shuls fill, but they had to rent larger places, erect tents and hire rabbi/cantors to lead services. There were different prices for the "regular cantor" and the "hired" one. And in many communities, they didn't have a regular synagogue, with a rabbi, but they'd hire someone. And lots of capable and not so capable rabbinic students or just anyone willing and able, make good money for a few days' work.
I'd like to know, so please give reply in comments, thanks.