Why did they remove it when that was one of the main advantages and unique features of the console's OS?
Because they were crucified for TV TV TV entertainment features. So they went chasing that performance gap for games.
Sometimes Snap was laggy but it would be corrected with a reboot. It was very functional and used religiously by a smaller subset of consumers. They pitched a replacement of Picture in Picture / Overlay mode from Win10 Core OS for some aspects but that fizzled out majorly. So now you have no replacement for it and left with a lesser experience that has to be crutched through using a second device like tablet or PC.