Marowak struggles to outclass faster sweepers due to it's lower speed and no way to boost it. Not even choice scarf, since you shouldn't be using it without thick club. Typhlosion is weak to the popular stealth rock and has some slightly underwhelming speed and special attack, though it can make up for that with a scarf, it has troubles swapping in and out. Heatran is usually used as a scarf user on sun teams anyway. Kingdra isn't actually outclassed as a Choice Specs user, just as a Dragon Dance. It just struggles to find itself on teams thanks to the banning of SS+Drizzle. Tentacruel is a much more popular Toxic Spiker due to better defenses, helping it get up two layers. 99/83/87 aren't very good defensive stats. Actually, they're about average, but not amazing for a wall. Mandibuzz is outclassed by Umbreon, though it does lack a fighting weakness, it's at the cost to weaknesses to rock and electric. Luxray doesn't really have a way to recover the damage over time it gets from status.
Anyway, a lot of pokemon have niches. They may be outclassed by one thing, but there could be times where they fit on a team better due to team weaknesses or something. I had a team that that needed a spin blocker who could take out Starmie one on one, and could preferably use trick on walls I struggled with. I ended up using Rotom's normal form, even though that's terribly outclassed as both a ghost type and an electric type. There is also the time I used Zoroark over Hydreigon despite better bulk and more power, Zoroark had Sucker Punch and the ability too lure Skarmory, which my team greatly needed. Marowak's niche is just one of the most powerful EQ's in the game, and works well on teams with paralysis or trick room support. Typhosion is one of the only pokemon to get STAB Eruption, besides Camerupt and Heatran, though Camerupt is too slow and Heatran can only have it with a quite nature, also making it too slow.