It's a bit more complicated than that. They changed it because GW1 was unsustainable the way they were doing things. That's why they killed it in the first place.
Do you have any evidence for this? Namely, unsustainability.
It depends on what you mean by evidence. We can make a pretty good guess that it wasn't economically viable, especially given the poor sales of Nightfall. Everyone who was there at the time was surprised at how fast the devs were pumping out expansion after expansion because box sales was the only thing that was keeping them afloat. The content of the expansions itself was too much and too exponential in the long run. They added 2 (!) new classes per standalone expansion, which already started to stretch the devs' imaginations and the game. For example, the Dervish got wind magic instead of the Elementalist's air magic and those are somehow different. On top of that, they added skills for
all classes in all of the expansions, the skill count ballooned to more than a thousand (!) at the end of Eye of the North. There was also a problem with story content, they wanted to create unique bread-crumb-style quests for the characters from all expansions to the stories of all other expansions, so that would've become crazy kooky and absurd at one point.
What we officially have is the story about the dev meeting after Nightfall was released and how they talked about wanting to do things (i.e. project Utopia) that were essentially impossible in GW1. They were already making compromises with Nightfall (the lack of cinematic trailer for it for one). After that, iirc they have gone on the record saying some stuff about the economic viability of GW1 but I'll have to search for that. They were also in a perpetual crunch to be able to create so much content for the game.
After that meeting, they decided to create GW2 instead of continuing with GW1 as a long-term project. I think they thought players won't be happy with them scaling back the content for each expansion. They *could* have kept developing in a sustainable manner, like no 2 classes or new skills for *all* classes per expansion, using a hub to transfer characters from one story to the next as opposed to creating an exponential quantity of quests, etc., but ultimately decided GW2 is the way to go.