What "rules" in particular did she violate, Robert?
As far as I can tell a sitting president such as Biden can voluntarily decide not to run again, as he did, which then opens the field.
His decision came fairly late in the process, there was no time to have another primary to determine who should run, so the Dems got together and made the obvious choice...Kamala, the sitting VP.
I get it that you and your fuhrer, Herr Trump, are miffed that the orange oaf now has a formidable opponent as opposed to weak Joe Biden: cry me a river.