Of all the fashion trends I have ever seen, bell bottom pants were one of the coolest. They were absolute must-haves in the 1960's and '70's. Every teen and young adult wore them. To not have bell bottoms on was to be one of them- The Establishment- which was way uncool, man. 
Along with your bell bottoms you had to have long hair if you were a guy, you just had to. If you didn't, you were part of them. Talkin' 'bout The Establishment, man! 
Women wore hip hugger bells and those were hot, man, were they ever. Hippie chicks were groovy. 
And then......  .....just like that, it all ended. Groovy went out of style. Like overnight, man. 

But that's cool, that's alright, we figured at the time. Fashion never slept and those inventive clothing designers undoubtedly had introduced some (maybe even cooler!) trendy new stuff that had wowed 'em on the runways in the major fashion centers and was trickling down through the populace. This was the way of it. We all knew that. 
Bell bottoms being fashionable had run their course- for now. 

There used to be an ad for Jim Beam whiskey, I think it was, that showed how everything that was in style at one time went out of style then came back into style years later. Trends came and went in a circle. Clothing was: In fashion, Dated, Uncool, Wouldn't be caught dead wearing it, Totally forgotten, Resurrected, Gotta have it at any price 'cuz it's the hot 'new' style. 
The ad was very perceptive and true to life for I saw this cycle play out with most apparel (and hairstyles). At one point early 1960's bowling shirts and shoes became fashionable again. Crew cuts. Capri pants. Bermuda shorts. Thick plastic framed glasses, or thick plastic frame glasses rimmed with wire on the bottom of the lenses, circa 1955, like Mr. Whipple wore (the "Don't squeeze the Charmin!" guy). Gaudy plastic sunglasses (Ferris Bueller's Day Off).
Beards came back. Big, bushy mountain man beards. Ponytails. Platform shoes. Elegant 1940's dresses. Satin sleepwear. Fedoras. Denim shirts. Paisley shirts. The list goes on. But never have bell bottom pants returned. 
Oh, women still wear them from time to time, they're around for that gender. Not as ubiquitous as they used to be, and not always in denim fabric, but they still make bell bottom pants for women. But for men? 
Ha ha ha ha ha! No man would be caught dead wearing bell bottom pants, far as I can see, even if they made 'em for men anymore, which they don’t. Straight leg is all that's on sale, which is the lamest look. 
If you wear Western jeans, you might be able to get away with something called 'boot cut', but that ain't bell bottom. 'Boot cut' is a slight flare, jest enough to get yer boot covered an' no more, 'cuz if'n it were more, you'd be gay or something. 
  "Ain't that right, Jeb?"
"Yessir Clem, that's fer shore right. I'll agree with you there". 

Bell bottoms existed way before Brokeback Mountain and I can guar-an-tee you, gay did not enter into the picture. At all. 
Bell bottom pants were the funkiest, coolest things men ever got away with wearing but some fashionista somewhere decided that men would be better off wearing straight leg jeans, which supposedly came from the punk rock scene, you know, the post-industrial, Euro-trash, gaunt and haggard junkie/squatter look.
Ug-ly! 
Back in the day, only geeks and nerds wore straight leg pants. Hep jive cat laid back dude hippie brothers wore bell bottoms. Shaft wore bell bottoms, and Shaft was the most Right On! brother ever to set hisself behind the wheel of a Lincoln Mark V, which was a boat of a car, so righteously big that you almost needed the Coast Guard's help to park it. 
The things young people don't know these days! You don't know of the rich history behind some articles of clothing and the options that could be available to you! I am so continuously amazed by this. People nowadays will cover their entire bodies with tats, shave their heads bald, wear piercings everywhere, choose nearly every conceivable piece of clothing ever known to humankind to create an 'outfit' with and not only that, they will wear it with pride. They will even walk around in florescent orange, green, and pink shoes and think nothing of it but men will not wear bell bottom pants!

  Why not?!

They seem to have no problem wearing everything else!