Because opinions. It's like arguing over which Resident Evil game, games, or eras of games are better. You can draw some logical conclusions, but there is a huge dose of simple preference, nostalgia, and beliefs about what a Silent Hill game SHOULD be injected into the mix. The mere suggestion that Silent Hill 1-3 have flawed and broken gameplay induces rage in some of the more "passionate" fans. The kind who sent death threats over the less than great remasters, for example.
The basic difference between Silent Hill and Resident Evil is that "modern" Resident Evil has flourished with games that sold gangbusters even if fans and or critics resented them, while "modern" Silent Hill kinda staggered along, and died. The series never attained the commercial success of Resident Evil, either.
Anyway, Silent Hill 1-3 are fairly consistent games. Graphics got better, gameplay stayed more or less the same, they were all "Silent Hill-ish". Sales fell with SH3, kinda like how they fell with Resident Evil 3.
Silent Hill 4 was a departure. It's basically 1408: The Game. Tank controls were abandoned for 3D movement akin to everything from Mario 64 to Haunting Ground to RE2 N64 & RE6. A huge chunk of the game is a very iffy backtracking section with an AI companion you have to babysit constantly if you don't want the bad ending(s). The game feels rushed, and the aforementioned backtracking is crushing for a lot of players.
Silent Hill: Origins followed. Development was a huge mess. Initially developed by Climax's American branch, the game was removed and handed to Climax's original UK branch, where Sam Barlow and his team went about trying to salvage it. Konami insisted they cut costs, so the game couldn't be completely scrapped and rebuilt. I think one element that had to be kept was the prerendered FMVs, meaning there's a bit of an odd tone thing going on between the FMVs and the ingame cutscenes. It's a strange game. One interesting design element is how it uses the method for handling fixed camera angles+3D movement that RE2 N64 pioneered.
I haven't played Silent Hill: Homecoming, so I can't comment on it.
Silent Hill: Shattered Memories is very interesting. Developed by Climax UK. Sam Barlow was lead developer. It started out as a very different game. The actual nature of the game is very complex because it was crafted to exploit a Konami loophole. Basically, Konami had on their books an opening to make a Silent Hill remake. Well, Climax weren't interested in remaking Silent Hill, but they wanted to make another Silent Hill game. So they PRETENDED to remake Silent Hill. They gave their characters the same names as the original characters to mask the deception, and rejigged the script so it bore vague resemblances to the original Silent Hill, despite having basically nothing to do with it. Shattered Memories is a game developed by someone who not only hated classic Silent Hill combat, but didn't like combat in general. (His latest project is the FMV game Her Story.) Shattered Memories reflects that because all you can do is run away.
The game was originally designed for Wii, and was ported to PSP and PS2. The Wii version is visually superior, but it has Wiimote controls, and I hate them so, so much. The normal flashlight pointing stuff is fine. Performing waggling motions to shake monsters off is not fine. I find the Wii version unbearable, and find the other versions to be far more enjoyable games. The PSP version is a technological marvel.
Haven't played Silent Hill: Downpour for the simple reason it never came to PC. Nor have I played Silent Hill: Book of Memories.