Seriously? What made you change your opinion?
The reasons are many, but it boils down to 2 central problems which the rest center around.
1. Like any Conspiracy Loon, the UFO subject is not about UFOs, it's almost always about the PERSON WHO SAW A UFO. That or the person's pet theory (like underwater dino-lizard men). I sat through over two-dozen MUFON meetings in the 1990s where abductees and contactees told their amazing stories, but when question and answer time arrived they were vague on details other than their fantastic story, and if someone (me) pressed too hard they would be ignored.
If you notice, almost every famous UFO incident has an advocate who's written books on the subject, and does radio shows, and conventions.
Actual UFO incidents, and I mean Unidentified Flying Objects, don't have an advocate, and tend to live in the news papers for a week or two before folks move on with their lives.
2. Almost nobody at MUFON, at least at the meetings I attended, was interested in collecting hard data, and getting to the bottom of a specific incident. It was all about belief, and not about knowledge.
In fact, within the UFO community, I was part of a sub-group called "Nuts & Bolts" because people like me were hung up on getting evidence...you know, physical evidence like soil samples from a landing site or whatever else someone could bag and take to a lab. The "Nuts & Bolts" folks usually get run out of meetings because we suck the fun out of the room...
...leaving just the Nuts...
I came to the conclusion that if the USAF actually had recovered alien space craft they could park them on the runway at Nellis with big neon signs that read "Captured Alien Space Ships!" and MUFON would scream "Disinformation!"