To be fair... you use a mobile phone in portrait mode usually as well.
Only my theory here but I guess widescreen SATNAV has been the norm for so long it as it was based on a small TV screen (after all, you could watch TV on them in the early versions).
Now that Smart mobile phones have become the norm over the last 13 years, it seems logical Volvo have gone that way.
Saying that, it will also take some getting used to for me too as the screens in my car are both very widescreen!