OLED costs more than LCD, significantly so, so to keep costs down they used LCD. Pretty simple really. OLED also has various issues that LCD doesn’t have that mean it won’t last anywhere near as long as an LCD screen does.

I love OLED screens, but after my $3k LG OLED tv started showing the PUBG UI every time there was red on the screen where the UI was, I’ve switched back to LCD for gaming and am not regretting it.