Tritium is useful only if you can manufacture a fission weapon in the first place. The vast majority of anti-proliferation efforts are aimed at preventing fission weapons capability. Preventing a nuclear weapons state from graduating from fission weapons to boosted-fission or full two stage thermonuclear is a much lower priority.

You can build a bomb with plutonium but no tritium, so if you want to prevent countries from going nuclear that's what you need to stop.

Also, the plutonium production is the bottleneck. E.g. this article about the Pakistan nuclear weapons program[0] estimated that the 50 MW nuclear reactor in Kushab could be used to produce 10kg of plutonium and 100g of tritium per year. The plutonium would be enough for about 2 bombs, while the tritium would be enough for 20.

[0] http://nuclearweaponarchive.org/Pakistan/PakArsenal.html