Bicycle-oriented navigation devices tend to use the portrait orientation. Net searching for images of e.g. Garmin bicycle computer, Wahoo bicycle computer, and basically any other minor brand gives you devices with their screen exclusively in the portrait orientation, both those that can show a map and those which cannot. There are also smaller square-shaped devices, and it obviously makes no difference to rotate them.
I failed to find a single bicycle navigation device that is offered in the landscape orientation configuration as a default. On the contrary, car GPS-navigation systems from e.g. the same Garmin are predominantly in landscape.
A smartphone is often used to navigate on the go, when a user holds it in one hand. The landscape orientation is more comfortable when two hands are used to hold a phone, and that is not always optimal when one is walking. As such, it makes sense that navigation applications are focused on single hand operation and related experience.
It is not exactly clear to me why there is such a uniformity in the designs, except for a slightly (possibly undetectable in practice) more aerodynamic configuration the portrait mode offers. Another thing is that handlebars have limited width (especially drop bars which are typically no wider than 48 cm), and everything that is placed on them or near them takes space from hand positions. The landscape orientation is clearly disadvantageous in this case, as the portrait configuration is "narrower".