Why has Wilder never tried to unify?

Oh ok.Did Wilder v Joshua get ordered as a unification fight?Genuine question as I’m not clued up on that stuff.
Also what are the rules re this?I know politics often stop this thing happening but is it like having to face a mandatory opponent?
I’ve said before my opinion is that both didn’t want the fight till later on and just didn’t expect one of em to lose in the meantime.

 

I don’t blame Wilders team for not fighting Wlad.No way was he experienced enough for Wlad at the time.What gets folks backs up is how long it took for him to fight anyone.That said he’s made up for it of late with Ortiz,Fury and Ortiz again.