Because she has not had a album to sell to the designated benchmark titled diamond. I mean it's like a first grade question. Not sure why feel a need to ask on a forum.
Same reason as why others don't. They don't. Comparing them to anomalies such as Adele and Usher (who can't even go Gold now) makes no sense. Sometimes things catch on. She's still considered the biggest thing in music without one (Harvey literally just said this a couple hours ago on TMZ). Doesn't matter.
Now it's obvious that this is a trolling thread. Many legendary acts have never had a Diamond album It doesn't hold weight as to if an artist is legendary. If that was the case, Britney would be a legend. And we all know that isn't fact. She's a pop culture icon though.