The reason is this.
Sony phones run a UI that's very close to the base AOSP code. It's not quite like a Pixel, but it's pretty close and the only major obvious customization are the icons along with some extra apps.
Samsung phones by contrast run a heavy UX layer on top of the source Android code. It looks and feels pretty different from stock but recent versions have at least someone retained familiar elements. As a result, it will take longer to update when it comes to OS versions.
Samsung flagships usually receive their updates in the middle of Q1 of the next year after the OS is officially released by Google.