Because one doesn't directly contradict the entire theme of the series.
How does that contradict the "entire series" (which at that point was only DS1 and 2)?
I really liked the dreamlike, melancholy atmosphere of Dark Souls 2. It had a lot of character. Love the Herald. Dark Souls 1 is the better game, yes. But Dark Souls 2's only lore sin imo was that it was TOO vague. But again, worked for the dreamlike, ethereal feeling of the whole thing.
Also how does it make you feel then that Dark Souls 3 is very much taking what Dark Souls 2 did and running with it? We've got the giants, the intro stating outright the lands of the lords are "transitory", which fits with Dark Souls 2 perfectly, the fact that there were many, many lords of cinder who lit the flame (or didn't, because it didn't matter)...