All characters from SH (1999) return in Shattered Memories but are often far removed from their original counterparts (doubled down that they change depending on your psychological profile)
Complete overhaul of gameplay
Complete removal of ongoing metaplot that spans from Origins to Silent Hill 4: The Room (will not elaborate for spoiler reasons but this is a big problem for me)
Many differences in how the town itself is presented
Massive differences in the Nightmare world, it's origins and what the purpose of it is
I feel that SM should be enjoyed as a separate entity from the series, I wouldn't recommend it as a starting point to someone trying to get into the series because it's so far removed from the canon plot. I think it should still be played because the mechanics alone are so cool. Problem is that I am so far up the original's ass as a source of comfort, I can't enjoy it more than the original.
As for porting issues:
There's a number of reasons that Konami just...won't. It's not an uncommon practice for developers to delete source codes for titles in the 1990s to 2000s, so the original source codes for Silent Hill (1999), Silent Hill 2, Silent Hill 3 and Silent Hill 4: The Room are not readily available. The infamous Silent Hill Collection released in 2012 was the zenith of their laziness and set the standard of why Konami just won't.
Bluepoint used retail copies to rebuild code from older games for remakes (as seen in the stellar release of the original Metal Gear Solid games) Konami however did not take on Bluepoint to do this for Silent Hill 2 or 3 for the collection rerelease - they simply handed it off to a inexperienced developer and used Alpha builds instead of rebuilding the assets like Bluepoint would. This led to frankly a disgusting rerelease that embarrassed the company and their fans (going as far as removing FOG from the rerelease of 2 to clean up the visuals even though the fog apart from being iconic also provided a means for draw distance loading). That's not without even touching how they didn't contact the original voice actors out of laziness to have them available for the relrease (meaning that everything needed to be revoiced) and literally using the wrong sound assets in the wrong places (i.e walking on grating in Silent Hill 3 used MACHINE GUN ASSETS instead of walking on metal).
It got awful scores and that just made Konami doubledown on 'well no one wants this, so we're not going to bother, we're going to invest all our money in our Pachinko machines'.
They're singing a different tune with this awful Silent Hill 2 remaster they've got cooking from Boober Team Bloober Team, they're in talks of getting the originals ported again because they're investing money in all of the new releases for 2024~ so Silent Hill 2 Remaster, The Short Message came out recently, Ascension (... :CowboyWokeVomiting:), Townfall and f (this is the one that I'm actually looking forward to, it's being written by Ryukishi07 of Higurashi fame and I have really high hopes for it).
In the case of Shattered Memories, it's also laziness but I think that it also comes in part that it's so tied in with how the Wiimode works that it would be difficult to find a suitable system to port to. I own it on all three systems (Wii, PSP and PS2), the PS2 is fun but it definitely doesn't feel the same as the original for the Wii. They might eventually get around to just using the PS2 port if everything works out with these new releases. Imo if Remaster and all the others flop, they won't chase the idea of porting anymore.
They might try porting it again if all of their investments over the next few years get traction, here's hoping they don't cut corners like they did the first time.
Thank you for coming to this terrible TED talk!