Weapons grade nuclear material is a whole different ball game from the merelly highly radioactive material needed to merelly make a dirty bomb.

A bomb made with the stuff that’s not “weapons grade” won’t reach critical mass so there’re isn’t a nuclear detonation - all you have is a conventional bomb that spreads highly radioactive material.

With the weapons grade stuff you can actually make a nuclear bomb (rather than merelly a dirty bomb), so something powerfull enough to wipe out a city.

So weapons grade nuclear material is much more tightly controlled and way harder to get your hands on that merelly highly radioactive materials. Fortunatelly it’s also way harder to make (you need either a breeding nuclear reactor or special equipment to separate the U-235 isotope that can be used to make a nuclear bomb from the much more common U-236 one which cannot from uranium ore, and since these are two isotopes of the same heavy element, they are very hard to separate, hence all the talk about special “centrifugues” in nuclear weapons programs).