>>88768467Because it's 'old fashioned'. Text boxes are the new thought bubbles. There are three main parts of it that I've picked up.
First, more faith in the artists. Text boxes were originally necessary because, let's be honest, most comic book artists weren't that great at getting things like emotions and action across on the written page. They were still figuring out the rules then, and writers were used to illustrations being supplementary rather than going hand-in-hand with their text. Or, in some cases, the text supporting the art.
It's been a long evolution on that. Some of the Golden Age books have narration in every single panel, while, as you move on to the Silver Age, the artists get better and the amount of narration shrinks. Some of the Lee/Kirby comics have half as much narration as the old WWII-era Kirby Captain America comics.
Second, too much reading. Comics are a textual genre, but there's a perception that people have shorter attention spans than they used to. The thought is that too much text on a page bores them and they end up just looking at the pictures and figuring things out from there.
Thirdly, the dominance of decompressed storytelling. In older days, you could resolve a fight scene in two or three pages. Now, they might take up half a book or more. Popularized by Warren Ellis during his time writing the authority, and used heavily by the likes of Brian Michael Bendis, decompressed storytelling allows everyone to get the nitty-gritty on everything, so rather than go "And so Miles told his friends what happened with Carol and Tony" you can spend half the story on flashbacks, things printed in other parts of the storyline, and rehashing what came before.
Used properly, decompressed storytelling allows for closer character examination. However, it's more commonly used as padding so the writer can spend a full page on something that could be summed up in one or two panels, getting more pay at the cost of less plot development.