Comics already possessed a strong visual storytelling format. They combined imagery with text, making the next step animation. It’s also for people who don’t want to read or look at a comic to understand the story ( more visual). This visual similarity also made it relatively straightforward for animators to bring these stories to life.
Secondly, comics were immensely popular and widely accessible. They had a broad audience base, which studios could capitalize on by adapting familiar characters and stories into animated shows/movies. But this often ends in failure as seen in the movie death note, Dragon Ball, Avatar: the last air bender, all popular animated shows or comics turn to garbage because of a quick money grab.But it is possible for them to make good adaptations of comics or books as seen with Netflix Avatar: The last Airbender, Avengers: infinity war/endgame Spider-Man, Harry Potter, etc.
