It's about the user bases - Luma and Partiful are almost entirely professionals in careers like Tech, Finance, or Entertainment (especially LA), and the events almost always vet before accepting people.

This helps ensure a better noise to signal ratio that Meetup simply couldn't provide.

Interesting point, but I personally didn't find Meetup had a noise issue. You could filter for the right stuff, pretty easily. Also I don't see how Luma/Partiful will avoid this problem eventually.