DPS classes have it easy. Their only goal is to deal more damage than the other guys. Their existence revolves around a single, immutable metric: DPS. There's no ambiguity when comparing two damage classes, as their DPS speaks for itself. As a DPS player's gear and skill improve, it directly increases their damage done, allowing them to evaluate their performance clearly and instantly.
Evaluating a healer is much more difficult. As their group's damage and skill improve, their healing numbers will actually go down. Healers are relied on the most when a raid is attempting a new encounter and gradually become marginalized as the fight moves toward farm status. As a healer, your best HPS performance might be the very first time you down an encounter. If you're killing heroic Ultraxion in four minutes, your raid simply isn't taking enough damage for you to parse highly. In order to properly evaluate a holy paladin's play, you have to dig deeper.