Planeswalkers have been around since Lorwyn, so it’s been more than 10 years. There’s 107 of them currently in circulation. During that time with that many planeswalkers, some of them have changed their outlook to life. Sometimes quite radically. They are major characters in the story, so it should be expected of them to have some sort of an arc, which might affect them mechanically as well.
We did learn in Amonkhet that the colors of a planeswalker doesn’t necessarily limit what they can do, because Nicol Bolas doesn’t seem to have any limitations on colors of spells.