The main reason for not having yet a duets album has to do with royalties issues.
Splitting and sharing royalties in such situations can be a very tricky and difficult task for both sides.
Firstly, people who manage Michael Jackson's music have to get the proper copyright licenses from other record labels.
Secondly, the artists (from those other record labels) if they like the songs then they also have to give their consent for such a release.
Aside royalties issues, there is also another thing: a duets album typically sells harder than a normal album, and this is a consideration that record labels always take notice of.
Lmfao, what?
Sony and the estate secured posthumous duets/features by Akon, 50 Cent, Lenny Kravitz, Justin Timberlake, Justin Bieber (presumably), D’Angelo, and Mary J. Blige with apparent ease. Royalty divisions also extend to production and songwriting credits, and they had no issue retaining Teddy Riley, Neff-U, Ryuichi Sakamoto, Mark Ronson, Tricky Stewart, Burt Bacharach, Timbaland, StarGate, Rodney Jerkins, Paul Anka, Babyface, or any of the other contributors to
MICHAEL and
XSCAPE. Royalties isn’t an issue.
Also, when you say “sells harder,” do you mean “won’t be as successful”? If so, that’s ignoring the fact that the Timberlake version of “Love Never Felt So Good” accounted for around 80% of single sales and propelled MJ back into the Top 10, which the Drake “duet” of “Don’t Matter to Me” replicated a few years later. Setting aside the authenticity issue, placing a current artist with MJ is a guaranteed moneymaker.
We don’t know why the long-discussed duets album hasn’t happened yet. In all likelihood they just haven’t actively tried to put one together.