Delibird is a lost case. The moment it got a Hidden Ability that was just its normal ability with another name, it's clear they will never make it even not absolutely atrocious.
On Deoxys's case... well, it's a Gen III Pokemon and Regenerator is a Gen V ability. Same with Pokemon like Empoleon getting Defiant instead of Competitive (which is the same thing except better for it).
Now, Deoxys could have had Regenerator as a Hidden Ability, but for some reason... it has none.
They've changed Abilities before, admittedly for balancing reason but no reason they can't change an Ability because newer more fitting Abilities have been made or the Pokemon just had an unneeded Ability.