Because substantial amounts of tritium are essential for "boosted" fission nuclear weapons. Tritium (and deuterium, which is cheap and easy to procure) adds fusion to the core of a fission warhead. It's not significant in energy production directly (unlike a true H-bomb) but it substantially increases the efficiency and potency of the fission reaction by adding a boost of neutrons at the moment of maximum compression.
It is considered essential to producing warheads which are small enough for militarily capable missiles.
Hackers were hence probably DPRK.