D4:Why can't 2 PC players play with crossplay turned off?

Whats is Crossplay?
It stated cross play from other platforms. but 2 PC, WINDOWS players require crossplay ON to play? any logically explaination Blizzard?

You’re posting on the Diablo I forum.
Perhaps you should ask here…

thanks mate, I was searching it on the list from Blizz forum home page, only saw this.