Standard Resolutions?

Why is it that LoL art is seemingly never available at a standard resolution? The seemingly rare times it's available at a standard 1080/1200p resolution is nice but why're they almost never available a standard resolution above that? For example the Lunar Revel promo site has 3840x1080 which is super weird, but lacks 3840x2160 or even the extremely common 2560x1440.