To answer this, I'll need to explain the context of marriage here in Brazil. It's a bit of a mess.

First there's the "proper" civil marriage (casamento civil): the couple picks a place to celebrate, requesting the presence of a judge, scribe, and two legal witnesses. The judge leads the ceremony and bang, they're married.

There's "stable union" (união estável), a couple living together in order to form a family without making it legally official. They already enjoy most benefits and responsibilities of a legally married couple, such as succession rights, but the property system is a bit of a mess. (i.e. legally it's undefined if their property is to be considered individual, shared, or shared only if obtained after constituting a couple).

Then there's religious marriage (casamento religioso), led by the religious figure the couple chooses. On itself it holds no legal power, but if the couple submits some paperwork in a registry office (a bit before or after the ceremony), it counts legally the same as a civil marriage.

Now, actually answering the OP: I think most people here go with the stable union route (instead of actual marriage) because it's less fuss, less expensive, and they already enjoy most legal benefits of a married couple anyway. So why bother? Plus the property system only matters if they split, and almost nobody thinks in this case.

It's also possible they performed a religious marriage but didn't bother with the legal paperwork; legally speaking it ends the same deal, as a stable union.