In the novels, the world of the ghosts is detailed, with complicated relationships that affect the storyline in later books. Moaning Myrtle, the Bloody Baron and the Grey Lady all play important roles later in the novels where extra characterization would have made the films stronger (in my opinion). Peeves himself is a worthwhile sideline to the "getting rid of Umbridge" storyline, but only if his trickster attitude and kinship to Fred and George has already been established in earlier books and movies.
"Cackling madly, [Peeves] soared through the school, upending tables,
bursting out of blackboards, and toppling statues and vases. Twice he
shut Mrs. Norris inside suits of armour, from which she was rescued,
yowling loudly, by the furious caretaker. He smashed lanterns and
snuffed out candles, juggled burning torches over the heads of
screaming students, caused neatly stacked piles of parchment to topple
into fires or out of windows, flooded the second floor when he pulled
off all the taps in the bathrooms, dropped a bag of tarantulas in the
middle of the Great Hall during breakfast and, whenever he fancied a
break, spent hours at a time floating along after Umbridge and blowing
loud raspberries every time she spoke."
A description of Peeves' behavior after Fred and George's departure
However, when the first movie began filming, many of the long-term implications of the ghost characters definitely weren't published. It's questionable if J.K. Rowling had fully planned out their importance, especially enough to convince the filmmakers to fully develop the characters. It makes sense to cut Peeves if he is only ever a source of commotion and a minor plot catalyst, which is the role he played in the first two books.
Based on the same set of information, fully fleshing out Nearly-Headless Nick makes sense though. He welcomes Harry to Gryffindor, establishes just how magical the world is, and is a major part of the second book's storyline. Not only does he have some emotional moments with Harry, but he is essential to the development of the plot. He gets Harry to the Death Day party, which is visually interesting to film in a book that doesn't have as much action early on as others in the series. His petrification helps Hermione identify the monster and is an important emotional beat, being the most easily recognized of the victims before Hermione, and he has several other major events. As that book was published in 1998, three years before the film release of Harry Potter and the Sorcerer's Stone, the filmmakers would have known that his character needed to be developed for the storyline to proceed smoothly. Peeves is never so essential, particularly in the first several books.