Michael, Madonna, Bruce Springsteen, Prince, Guns N Roses, U2, Whitney Houston and, ahem, Phil Collins.
These are some of the names that received Diamond-certified albums in the 1980s, arguably the most iconic decade for superstars. All these names are iconic household names who were pop culture phenomenons.
Yet there is a name that is missing there, Miss Janet Jackson, who has a similar level of influence on subsequent artists.

(not a Diamond award)
Yet why did she never get a Diamond album? What held her back from reaching that stratosphere of album sales at the peak of her career?
I'm geniunely curious, because Janet is certainly comparable to many names there and had hit after hit after hit. For example I'd like to see anyone argue that Phil Collins has more impact than Janet, but he had much bigger sales.
What held Janet back?
Discuss politely.