Mainly because the Xbox one X is 3 years old now. On paper the Xbox series s has better performance than the one X but it almost feels to me like Microsoft have intentionally nerfed the system so it doesn't match an Xbox one X? Microsoft should have bumped the price up slightly and added more memory so the series s could run existing titles in 4k.
I think what frustrates me personally is that if I bought a new Xbox series S I'd end up running existing titles like halo 5 and Forza horizon 4 at the same 900p they ran on my 2013 xbox one.
And while 1440p upscaled to 4k looks pretty reasonable 900p on a 50" 4k TV looks like garbage honestly! Clearly MS must figure that having the cheapest "next gen" system is their priority though and will work best for them. They might be right as many people are fearful for their jobs and probably don't have much disposable cash at the moment so perhaps it's the right system, right time at the right price.
Time, and Sony's imminent PS5 pricing announcement, will tell!
The architecture in the SS is a lot more up to date and it will perform well - but its optimised for 1080-1440p. Thats a design choice to have a more inclusive next gen.
I'm not sure expecting 4k support at this price point is reasonable. MS have made it as reasonable as possible to get a SX via All Access too.
Perhaps just get a SX if you need 4k? Or if you don't want to pay for it then just accept that you will need to live with the limitations of the SS.