Likely because we tend to see hints in clouds, colored chairs, and comments on eye strain.
I see you like to keep making fun of these hints, but never bother to give a reason besides "it's a stretch" when it really isn't considering that, so far, they've all come true.
I'd like you to give me an explanation why you think the barrel and wooden ladder were in the background if they were not hinting at K.Rool.
"It is just how his office looks!" No, this stuff is way too random for that to be the case, it has to be a stage.
"It doesn't mean anything!" No, there would be no reason for the background in the 8/8 Direct to look any different then. Changing a background that has no hidden meaning would just be a waste of time. Besides, if I remember correctly, the background in the Smash 4 Directs were always the same, so why bother changing that now?
It also boggles my mind how anyone can argue that the "red eyes" comment was not a hint. Seriously, both "real" newcomers in the Direct had a connection to red eyes. Simon with Bloody Tears, which just so happens to be one of the most popular Castlevania tracks ever, and K.Rool is obvious. Yes, Sakurai really had red eyes in the Direct, but that doesn't disprove that this was also a hint, unlike people seem to think. There is no reason for it not to be both a hint AND him saying he's tired.
I think it's not possible for Sakurai to give hints that are less subtle without just completely giving it away. There will also again be someone to say "well then the plant and floor must also be hints in your eyes lul". No, because, unlike a freaking rope ladder in an office, those don't stand out.
Seriously, if you sent someone a picture of the background and told them that it's an office and nothing else, almost anyone would question a ladder like that just hanging there.
I genuinely want a real argument as to how anyone can think what I said here is not accurate, besides "it's a stretch" and "you're looking too much into it".
I don't even want you to believe the chairs mean anything, even though they likely do, but these past hints were, undeniably in my eyes, hints.