Think about phantom assassin for instance. Her passive skills are apparently beneficial for a carry, her blink and dagger lets her close the distance. Her dagger also has a damage, but that's not important. When you look at a hero, whether it can carry is determined by the scaling factor of its skills. Almost all skills of carries have impacts on the combats that scale with the carries, and their items.
Then look at pit lord.
His first skill is only relevant in the late game because of the percentage damage. It doesn't make him stronger, or enemies weaker. It's just damage, scaling with enemies' health.
His second skill is a good spell, since it provides crowd control so it's easier for pit lord to fight in combat. It's the only other spell relevant for a carry.
His ultimate is just irrelevant for a carry. He already has a good utility spell, and he wastes one more (ultimate)slot for another utility spell which provides nothing but positioning.
Moreover, he is a strength hero, which has only 12 base agi and 1.3 agi growth. He won't even be able to utilize the damage he gains with his third skill, because he can't hit fast enough.
He will be just a worse version of Tiny, nothing more as a carry.
But this shouldn't stop you from playing him as a carry in pub games. You should try to make up for speed, buying items like s&y, assault, mjolnir. Buy a bkb, you are not very mobile. Armlet might be good for midgame. Abyssal for an additional bkb piercing stun.