It might have been in order to avoid a support ledger attached to the house. Is the siding not interrupted by the deck? That would allow water to drain properly at the face of the house.

I use Maine Deck Brackets to achieve the same thing.

Almost all old house porches I've seen had the porch boards perpendicular to the house, which allows for draining along the length of the boards, and not over the joints to other boards. Fewer end joints.

Drainage makes sense - most of the other old houses in the neighborhood have porch boards perpendicular to the house - but our porch (unlike almost all the neighbors’) is fully enclosed, and during the renovation all the evidence suggested it had always been enclosed. Maybe it was built the same way as the open porches out of habit, even though the drainage issue didn‘t apply. Or maybe they just followed the pattern of the house joists (which like the porch joists run N-S).

If the porch boards are perpendicular to the house then the supporting joists would be parallel to the house.