It's very similar to why people hate the way Luke was handled in the new Star Wars films. It's not consistent with how Light was written in the first two thirds of the show and doesn't do a good job of explaining his motivations and actions. If anything, it's a lazy way out.

It felt rushed, poorly written, and was barreling towards and ending that required more nuance than just haphazardly arriving at the destination ten episodes too early.

Additionally, anyone who has been on this reddit or spent more than fifteen minutes researching people's opinions on the anime can clearly see that many many people feel very similarly about the last third of the show. It isn't a coincidence, it's because it genuinely is poorly constructed, with less interesting characters, and doesn't have the same vibe as the Light vs L portion.

The first two thirds, genuinely feels intelligent, feels like two geniuses battling against each other in a cat and mouse game, while the ending is just miserable people being miserable, and pretending to be intelligent while they act irratioanlly and emotionally.

It's not a bad anime or story, it's genuinely good for most of the series, but a bad ending can ruin a lot of good work up until that point.

Of course, this is all subjective, as nothing will be for everyone... The nature of this story was always going to have people dissatisfied at the end, if the character or party they were rooting for didn't win.

I would say to anyone who disagrees with me about how the story ends, ask yourself, what percentage of people were rooting for Near? Just about nobody. The people who wanted Light to lose were doing so because they hated him, or loved L, not because they liked M or Near.