If your motherboard was replaced, the repair technicians need to 'enable' the keyboard backlight function for your replacement motherboard (unfortunately, there isn't a way you can do this on your end as far as I know), so it's possible repair needs to fix this for you if they forgot to do this.
There's also a (small?) chance the keyboard backlight is just defective though, but either way ASUS repair should be able to solve it if you want to look into this.
Hope this helps.