Dark Knight was a popular requested job. They also wanted to obviously add one of each role in Heavensward. Dark Knight was chosen although it ended up being a tank because MCH was the DPS. Look at how much Clemency steps on healer toes when it comes to HP management. They probably figured a tank with HP costs or mechanics based on absorbtion would be a little too variable for your average healer to deal with and difficult to design encounters around. It wouldn't bother skilled players much, but FFXIV is pretty accessible so they didn't want the average DRK to constantly be killing themselves with poor HP management. Thus, we got FFXIV DRK.

Instead, it seems like they tried to keep the whole theme of draining yourself to fuel your abilities via MP instead of HP. It still impacts (or at least impacted, in past expansions) your performance to a decent degree and you get the "theme" of self-draining but without the risk of killing yourself and pissing off your healer. I'm not necessarily saying I agree with that but it seems to be the path they took so that the job is at least in the slight spirit of the whole dark knight theme.

As for enfeebling and enhancing, they used to have stuff like this on other things. Slash/pierce/blunt resist debuffs, Halone's Rage lowering STR, WAR lowering damage dealt by 10%, the various abilities like virus and the other one that escapes me at the moment that lower STR/MND/DEX. I wouldn't mind DRK having this but odds are if they did in a game like FFXIV you would have to have other jobs that do these things too, and considering they removed it from PLD and WAR they don't want to go that route. The only things that remain that do this are the role abilities, most likely again so you get some small sense of a "support" ability even though tank/dps/healer trinity doesn't leave any room for support classes that FFXI's more flexible system did. Personally I loved red mage's role as a powerful enfeebler and jack of all trades, but you won't see that in a holy trinity style MMO.