Because Bono was young and naive then. U2's fan base was similarly (on average) younger and more naive. The two were aligned.
But as people grow up, they just want to hear the music without the sense they are being preached to. People change.
I get your point in general. Band and crowd aligned in age and viewpoints of the world- wide eyed, idealistic, etc.
However, I disagree that Bono was naive. He'd grown up working class, lost his mom early, had seen poverty, addiction, war- and developed a pretty strong, realistic grasp of most political issues by the early 1980s. People were always trying to interview him- in large part because they knew he could speak intelligently about music and many topics beyond music.
I guess what I'm trying to say is he was never just a naive youthful flame thrower who resonated with a fan base that was at the same stage of rebellion in life. I'm far from the final authority on this, I've only had one brief conversation with Bono when I was working security years back. However, I've read as much as any fan about his life and background and from my reading and from the accounts of others as well, it's actually remarkable how much of who he is was already formed by the time the band was well known.
I just don't see him or his views really changing that much over time. He was always a capitalist who thought his goals could be accomplished by working in the system. He was on a politically appointed commission on youth unemployment in Ireland in the early 80s. Always talked about meeting with world leaders, etc.
The only real change I've seen is he is more diplomatic , less preachy and more willing to work with people who don't share his views on most issues to accomplish things. It's pretty hard to be a political leader these days and get a good slap from him- you have to be as bad as Trump to get it just for what you stand for. Otherwise, you'd have to promise him something and then go back on your word like Stephen Harper did about 10 years ago.
I honestly feel like a lot of this is learned behavior from the U2/Bono hate that seems to have gained traction over time. The made up tax scandal colored a lot of people's perceptions- quite unfairly- of his work on behalf of the poor.
Fans in the same demographic still go and see acts that are far more controversial and direct than U2 in their politics and preaching and they don't utter a word of criticism.
Bono really puts himself out there with his work and advocacy. Who else has used their fame for as much good as him? That makes him more of a lightning rod.
I'm not discounting your theory- people certainly change- but I think in Bono's case, it's more due to the fact that there's this perception that you're supposed to think of him as an annoying, elitist hypocrite even if you love U2's music.
An elitist who spends a lot of his own time and money doing all kinds of work for people and never stopping despite criticism when he could've sat on a beach somewhere since 1987 and never ran out of money.
It just doesn't make sense.
It also doesn't help that Bono's center/left brand of politics has never been less popular in the politics of the world than it is now. For all of U2's career to this point, viewpoints similar to Bono's were at least strong and at times dominant. The far left and far right have gained a ton of traction even since around 2013 and both groups are about as hostile to Bono and his thinking as they come.
My take, and this long diatribe wasn't all a response to what you said, AndrewCowley. I think for a certain number of people who take issue with him now, you've hit the nail on the head. I'm just arguing that their perception and framing are off.