It was likely cheaper and more convenient especially if there was a child on the way. It might be useful to include this quote below from an e-mail I received several years ago from a Liverpool RC researcher as the island's RC churches come under the Liverpool RC Diocese:-

"Many Catholic couples married at C of E churches. They always baptised their children 'properly' but I think they considered marriage to be as much a legal as a religious requirement. Perhaps it was cheaper and easier to get married at a C of E church. At a Catholic church you would have to obtain a 'certificate' (instead of banns) and then pay both the priest and the registrar. At a C of E church three readings of the banns would suffice plus the vicar acted as the registrar.

Catholic couples marrying elsewhere was considered to be such a problem that the church introduced a new marriage law in 1908 making only marriages at Catholic churches valid. It is after this date that the practice of adding the information about subsequent marriages to the baptism entries were introduced. When the marriage parties applied for their baptism details the priest was meant to add the new information but looking at the registers its obvious that some of them were more efficient than others. The absence of marriage information doesn't mean any particular child didn't marry."