Because the priority for any carry is farm -> survivability -> damage. Without farm, you can't get more survivability or damage (through items). If you're dead, you can't farm or do damage. If you don't do damage, you don't carry anything.

Each hero needs these things, and depending which hero you have, you get these abilities either through items or skills.

Farm is either some sustain like health or mana regen or AOE damage like battlefury or maelstrom.

Survivability is health or armor/magic res.

Death pact is both farm (it instantly kills a creep, giving you its gold, and thus makes you farm faster), sustain (heal) and survivability (max hp). It instantly gives you better farming and better survivability, the first things you need as a carry.

The other abilities just don't do as much.