Publication ethics demand that authorship rights are shared among collaborators in particular studies. Since the PhD research was carried out with the supervisors, it is mandatory to include their names. Yet, the lion's share of the study was done by the student and as such s/he deserves the lead authorship position.
If the supervisor is in any way responsible for the research (the research was carried out under the supervisor's oversight, it was written as a paper for one of the supervisor's classes, etc.), then the student must acknowledge the supervisor in the research's publication. However, if the student completed the research entirely independently of the supervisor (let's say I researched Taoism and my supervisor only oversaw my work in Christian history), then the student has no obligation to (and probably should not) mention the supervisor in the research's publication.