Probably I was too lazy to post another answer. Previous one wasn't good so I hide it, here's hopefully a better one :')
Primal Kyogre just pales into comparison with Primal Groudon, because of the sheer difference in sets.
What does Primal Kyogre run? Standard Calm Mind Sp.A set, which is very easy to guess and thus wallable by Primal Groudon. On the other hand, Primal Groudon's sets are really varied—you can't guess if it's a special set, mix, Swords Dance or even Toxic Rest defensive sets. Primal Groudon is able to check a whole part of the meta-game—Zacian-Crowned, Ho-Oh, Kyogre formes, Necrozma-Dusk-Mane, Eternatus, etc. etc. with so many sets of it own. It can fit into any play-style, let it be Hyper Offense (Swords Dance Rock Polish Double Dance), Balance (Stealth Rock setter, Xerneas phazer), Bulky Offense with mixed sets as well. This makes it very unpredictable and hitting unsuspected foes very hard.
Unfortunately, Primal Kyogre's sets don't have any notable variation
- You just see standard Calm Mind with the most easily guessed moves being Origin Pulse, Thunder, Ice Beam, Water Spout, etc. meaning it requires very less amount of prediction skills to play around.
- Scarf Kyogre is also very good and devastating, but as ~Chocolatte~ told it's STAB move is nullified by Desolate Land, it doesn't have anything to hit it hard in return. Ice Beam is a guaranteed 4HKO on Defensive Primal Groudon while Precipice Blades is a clean 2HKO.
252 SpA Kyogre Ice Beam vs. 252 HP / 200 SpD Groudon-Primal: 97-115 (24 - 28.4%) -- 97% chance to 4HKO
0 Atk Groudon-Primal Precipice Blades vs. 0 HP / 4 Def Kyogre: 235-277 (68.9 - 81.2%) -- guaranteed 2HKO
Splash-ableness of Primal Groudon versus that of Primal Kyogre
- Primal Kyogre just limits team-building. You need a solid Primal Groudon counter in form of Arceus-Water Ground, etc. but that doesn't leave it alone. Primal Kyogre has more foes it has to compete with. There are things that can just outspeed and defeat it in one shot on the other hand you're aren't doing anything.
4 Atk Life Orb Dark Aura Yveltal Max Darkness vs. 0 HP / 0 Def Kyogre-Primal: 309-367 (90.6 - 107.6%) -- guaranteed OHKO after Stealth Rock
+1 252 SpA Life Orb Calyrex-Shadow Psyshock vs. 0 HP / 0 Def Kyogre-Primal: 333-394 (97.6 - 115.5%) -- guaranteed OHKO after Stealth Rock
+1 252 SpA Calyrex-Shadow Never-Ending Nightmare (190 BP) vs. 0 HP / 0 SpD Kyogre-Primal: 369-435 (108.2 - 127.5%) -- guaranteed OHKO
+1 252+ Atk Zacian-Crowned Wild Charge vs. 0 HP / 0 Def Kyogre-Primal: 432-510 (126.6 - 149.5%) -- guaranteed OHKO
252+ Atk Life Orb Rayquaza-Mega Dragon Ascent vs. 0 HP / 0 Def Kyogre-Primal: 391-461 (114.6 - 135.1%) -- guaranteed OHKO
- Now, what do you need? Arceus-Fairy for Yveltal, SpD Yveltal/Arceus-Dark for Calyrex-S, Necrozma-Dusk Mane for Zacian-C and a hard wall for Rayquaza-Mega.
- That's just a few of them. To build around Primal Kyogre takes too much space and no significant rewards in return. Your teambuilding will get entirely limited if you use Primal Kyogre. On the other hand, Primal Groudon helps its counters' check. For example, Primal Groudon takes care of Arceus-Fairy's weakness to Necrozma-Dusk Mane and Zacian-C. In return, Arceus-Fairy punishes Yveltal and Mega-Rayquaza enough. This way, Primal Groudon doesn't restrict teambuilding.
Competition from Life Orb Dynamax Kyogre
- A huge disadvantage of using Primal Reversion is that you can't Dynamax. A Dynamax Life Orb boosted Kyogre hits much harder than Primal Kyogre, so you may consider that instead of it.
- Using Hidden Power [Flying] as a new innovative tech. HP Flying turned Max Airstream can boost your average Speed to much better results. However a good question arises about why not using Yveltal.
- Choice item Kyogre are also not much viable than Primal Kyogre. Kyogre has a little higher ranking in VR (Viability Rankings) than Primal Kyogre but this is because of Life Orb Dynamax sweeper, not Choice items.
252 SpA Kyogre-Primal Ice Beam vs. 0 HP / 0 SpD Rayquaza-Mega in Strong Winds: 252-298 (71.7 - 84.9%) -- guaranteed 2HKO
252 SpA Kyogre-Primal Water Spout (150 BP) vs. 248 HP / 8 SpD Eviolite Chansey in Heavy Rain: 298-352 (42.3 - 50%) -- 0.4% chance to 2HKO
252 SpA Kyogre-Primal Ice Beam vs. 136 HP / 52 SpD Groudon-Primal: 130-153 (34.6 - 40.8%) -- guaranteed 3HKO
252 SpA Life Orb Kyogre Max Hailstorm vs. 0 HP / 0 SpD Rayquaza-Mega in Strong Winds: 411-484 (117 - 137.8%) -- guaranteed OHKO
252 SpA Life Orb Kyogre Max Hailstorm vs. 136 HP / 52 SpD Groudon-Primal: 212-250 (56.5 - 66.6%) -- guaranteed 2HKO
+2 252 SpA Life Orb Kyogre Max Geyser vs. 248 HP / 8 SpD Eviolite Chansey in Rain: 675-795 (96 - 113%) -- 75% chance to OHKO
The higher damage output of LO Dmax Kyogre makes it a more attractive choice than Primal Kyogre for non-Primal Kyogre nostalgic fans (including PrimalKyogre) . That's why it faces stiff competition and is deemed to be an awkward choice in many teams.
Hope this helps!