Gengar's paper-thin defenses and much lower speed matter a lot. It's not a bit bulkier as Darkrai has close to 50% more bulk on the physical side than Gengar does and has a typing that is better against the most common priority move in OU in Sucker Punch. Darkrai if dropped into OU would be the third-fastest Pokemon, which allows to beat a lot more than Gengar since many OU threats have more options for OHKOing Gengar than they do with Darkrai and are faster, such as Cinderace (Pyro Ball and also Sucker Punch if you can fit it), Choice Specs Dragapult (Shadow Ball and Draco Meteor whereas it can only OHKO Darkai with Draco Meteor), pre-transformation Battle Bond Greninja (Dark Pulse and Hydro Pump whereas none of its moves OHKO Darkrai), Meowscarada (Knock Off and Sucker Punch while it doesn't OHKO Darkrai with anything), and speed-boosting Walking Wake outside of sun (has a chance to OHKO Gengar with Hydro Steam whereas Darkrai lives). And as you yourself pointed, out, Dark Pulse is more spammable due to it having no immunities.
One of the bigger upsides Darkrai has over Gengar is that it beats Clodsire, the main special wall of OU, while Gengar outright loses to it.
To pretend that Gengar is even remotely comparable to Darkrai in performance is disingenuous or just outright wrong.