It’s pretty straight forward really. Rye maps are drawn from an elevated viewpoint of the town and, as such, if the map were to be drawn drawn in the opposite orientation, all the shops and attractions would be hidden behind the hill that Rye sits on. Which wouldn’t be very practical. Other reasons are that the church faces north, most of the roads and the railway come into the town at the north and, as such, the presentation to the visitor is much more palatable to them when the “entrances” to the town are shown from the bottom … as is done with almost all attraction maps.
Anyway, maps have been drawn of Rye like this for centuries, it’s quirky and we like it 🙂