Relic substat allocation

[Canadane]

If I can put tenacity on my deathlocke lux then I see no reason I should not be able to put direct hit on an axe especially after they’ve made it scale on guaranteed direct hits.

The reason is Tenacity is a tank stat, Direct Hit isn't, even if you could benefit from it.

[Hasrat]

The reason is Tenacity is a tank stat, Direct Hit isn't, even if you could benefit from it.

Deathlocke is MCH HW relic. So, as Tenacity is a tank stat, it makes as much sense putting it on MCH as DH does to put on tank weapons.

The actual reason is probably because more importantly, prior to 4.0, Tenacity didn't exist. It would've been Parry materia at the time, which actually would have had an effect on all jobs, which explains why relics prior to StB allow you to play more loosely with your substats.

Point being, OP, can't really compare relics/substats from older expansions, as stats and materia have been revised over time.

Why Direct Hit? Probably because, during the 4.0 revision, they realized they were creating a tank-only stat (Tenacity) and a healer-only stat (Piety) and decided to make a stat at least somewhat restricted to DPS.