Honestly, I dunno. Could be that it was on Saudi time which was inconvenient for the US PPV market (which were the only numbers ever publicly discussed) could be that things just naturally draw more when they happen under the UFC banner, could be that the ridiculously low numbers put out are BS propaganda to discredit Ngannou. Most likely a combination of those. I highly doubt it failed to get buys in six figures globally considering it involved Joshua as well, and looking at the amount of online and media attention it got.
But I mean...are you honestly arguing that if Ngannou had returned to the UFC still champ and having arguably just beaten the (at the time) consensus no1 HW boxer IN A BOXING MATCH it wouldnt have done a lot for his stock and by extension the UFC HW divisions? really?