They explained indirectly in the movie. First they say that the adamantium is finally overcoming his regenaration, and it is killing him. This is true. But why his regeneration lost against the adamantium? For The same reason the rest of mutant have vanished from Earth, because Dr.Rice and his scientists develop a toxin that they implanted on the food consumed by people that suppressed the mutant gene. All started with his new strain of corns, and the high fructose corn syrup that came out of that, the strategy they took to defeat them on the long run.
Hope this helps you, and we can continue have fun talks about movies we enjoy so much they become part of our lifes in degrees of meaningfulness!
Have a great day.