The reason for this is because such a maximum value is dependent on the equation of state of the matter composing the star. This thermodynamic equation describes the state of matter under a given set of physical conditions – i.e. temperature, pressure, volume, or internal energy. And while astronomers have been able to ascertain within a degree of certainty what the maximum mass of a nonrotating neutron stars would be, they have been less successful in calculating what the maximum mass is for those that are rotating.
In short, they have been unable to determine how much mass is needed before a rotating neutron star will surpass its maximum speed of rotation and finally form a new black hole. As Rezzolla explained:
“What made it difficult in the past to calculate M_max is its value will differ from what composes the neutron star (i.e. its “equation of state”) and this is something we don’t really know. Neutron-star matter is so different from the one we know that we can only make educated guesses; and unfortunately, there are many guesses because there are several different ways to compute the properties of the equation of state. So one ended up up with a situation in which not only the maximum mass was different for different equations of state, but even the maximum rotation speed was different for different equations of state.”
However, in their study, titled “Maximum mass, moment of inertia and compactness of relativistic stars” – which appeared recently in the Monthly Notices of the Royal Astronomical Society – Rezzolla and Cosima Breu (a Masters student in theoretical physics at Goethe University and co-author of the study) argue that it may now be possible to infer what the maximum mass of a rapidly rotating star would be.