Here's the thumbnail version:
"A Star is Born" premiered at 181 minutes, but then cut down by 30 minutes because Jack L. Warner thought a shorter film would make more money. The cuts were made without the approval or assistance of director George Cukor and these cuts ravaged the film.
In the early 1980s, film historian Ron Haver persuaded WB to restore the cut version. The multichannel tracks was long gone (and sadly, this was not the only WB film this happened to; Calamity Jane, Giant, and Auntie Mame are among the sad list of stereo films now only extant in mono), but a 151-minute print with 4 track stereo (with very few high ends) was located. A search for cut scenes began. They were able to find a mono 181-minute soundtrack, and looped in the cut scenes. For about 5 minutes where footage absolutely, positively could not be found, B&W stills were used instead. Later in the 1980s, supposedly WB got in contact with a former employee who said he had not one, but two IB Tech 35mm prints that were totally uncut, but the deal to acquire them fell thru. I suppose that when Time Warner merged with Warner Communications and with Ron Haver's death in 1993, the drive for futher work died too. Hopefully they will make one more try or another search through the vaults.
There are plenty of films that may benefit from the reinstatement of cut scenes, but those aren't the ones that get it, and the superfluous "director's cuts" often ruin it for them.
More discussion here:
Read Ron Haver's book on the making and reconstruction of the film. It's out of print, but I found a used copy on Amazon. I recommend it.