Crabominable has common weaknesses, like Fighting, Fire, and Psychic, that make it harder to survive hits than Vikavolt and Dhelmise. Dhelmise actually has good enough bulk and access to Rapid Spin to be a Main choice of Mono Ghost teams, which go against way higher tiered Pokémon.
You also have to consider Crabominable's Role. Vikivolt and Dhelmise are great defensive Pokémon, while Crabominable is a more offensive Pokémon, getting less EVs and generally no nature investment into it's Defenses. :P
Decent Bulk is 80 HP with 80ish Defense, although, it doesn't exactly work by averages, and bad defensive typings and EV/Nature investment can contribute. A Pokémon could have 255 HP and 5 Defense (an average of 130, Higher than defensive bulk Arceus's Average) and still be a Glass Cannon.
Defense matters a little bit more than HP; 80 of a Defense with 70 HP is generally better than 70 of a Defense with 80 HP. Here are calcs to prove this (I used no EVs, Items, Hardy Natures, Abilities, etc.):
90 Base Atk Test Pokémon Tackle vs. 70 Base HP / 80 Base Def Test Pokémon: 33-39 (11.7 - 13.8%) -- possible 8HKO
90 Base Atk Test Pokémon Tackle vs. 80 Base HP / 70 Base Def Test Pokémon: 36-43 (11.9 - 14.2%) -- possible 7HKO
As you can see, the Pokémon with less HP and more Defense took less damage.
Crabominable has a minuscule 67 Special defense, when it's weaknesses are more abundant in Special Attacks. It also gets most of its bulk from HP, which isn't the "greatest bulk". Crabominable is also slow, so even if it could take more hits, it would still be at a disadvantage always being hit first. For these reason, Crabominable isn't great competitively and usually goes down quicker than most Pokémon, but hitting hard before that, and thus being a Glass Cannon. :P
Hope I Helped!