Can I get a better explanation than "it is better"

Nope.

Have both and I strongly prefer the Demise.

Well that's your opinion. And that's ok.

ROF is too slow for it to be truly effective outside of PVE. Even if you land all your critical shots, you're still not gonna beat out the usual suspects, I.E., Thorn, Hawkmoon, the Mythoclast or Suros. Game speed and map size make playing the range game a limited proposition. Most efficient way to make the most out of Ghelion's Demise is to trail a few yards behind some teammates and mop up their kills, which nobody takes too kindly to. It's a fine trophy gun and I love the weapon in story missions or on patrol, but the trade-off between impact and ROF makes the Rapier the better choice whenever TTK is vital.