He was the only Bond actor to be let go. Connery left of his own accord, as did Lazenby (initially), Roger was okay with leaving as he knew it was time, and Dalton's contract expired (and he also got bored of the endless delays with what would become GE). Nobody likes to be told they're not wanted anymore. It's ironic, of course, that he was let go when Purvis and Wade, who were directly responsible for much of DAD's excesses, were retained almost indefinitely. If I ate a Mr Kipling cake and got food-poisoning, I wouldn't blame the drawing of Mr Kipling on the cover of the box! I wonder why he didn't complain about the DAD script at the time. I mean, this was supposed to be the fruit of three year's work! Some of it - like the bad CGI - wouldn't have been evident until afterwards, immedaitley, but the shifts in tone and the much-maligned invisible car...!
First, I'd heard that MGM made it very clear they didn't want Dalton to stay on, despite Cubby going to bat for him, so in a way he was "let go" as well. Or at least he felt the sting of being "not wanted" by the powers-that-were. The difference being, I gather, that Dalton always saw the role as a double-edged sword at best and when push came to shove he was perfectly willing to give it up in exchange for a return of his privacy and some artistic elbow room.
But the bigger picture that emerges from "The Brosnan Gripes" is that the whole thing challenges the conventional wisdom that the lead actor has some power over the direction of the series. Everyone assumes Roger somehow pushed the producers to play up the comedy for his benefit, and that Dalton used some kind of influence to make things grittier and more down to Earth. But if it were really true that the Bond-du-jour had such clout, surely the much-touted "Billion Dollar Bond" could have pulled strings to get his stated vision of Bond put on screen.
I think what we're seeing is that the films, rather, are built around an actor's perceived strengths, but not in acquiescence to his demands. Seeing Roger could do light comedy, the script-writers infused more humor, even though we know from other films (and even "The Saint") that Roger was quite capable of a harder-edged approach and indeed might have benefited from a push in that direction. Likewise when Dalton shone in the more dramatic scenes in TLD, the reaction was to write a screenplay that was ALL brooding and anger, so we got LTK, a pretty humorless (even dire) entry, though IMHO Dalton could handle humor just fine.
IMHO, the problem with writing for Brosnan is that he had no such clearly defined "strength." He wasn't "the tough Bond" or "the funny Bond" or "the dramatic Bond"...he was sort of an amalgam of all that went before, hard to define...a generic Bond avatar; handsome, charming, well-dressed and well-spoken, but in the end, as the old saying goes, there was no "there" there, nothing to hang your hat on as a writer. If Roger was hired to handle the humor (already in place with DAF), if Craig was hired to "shake things up," Brosnan was hired to "give the people what they want." He was hired to look good in a tux, be suave and handsome and run around with a gun now and then. After the tepid response to LTK, the long legal battles and the risk of irrelevancy from a 6-year-hiatus, what was wanted was a "no surprises" Bond, someone who could look and sound exactly as the masses expected Bond to look and sound, period. And so without a defined personality to serve as the gravitational center of the films, they become an exercise in throwing in all the Bond tropes, hitting all the marks expected from the tried-and-true formula. Brosnan, in his mind, should have been a tougher, more substantial Bond, but that's not what the producers and writers saw in him; that's not how they perceived his role on the team. He was cast, basically, to be a "standard Bond", an "amalgam Bond." That's what worked for him at the Box Office; playing to expectations. Why would the bosses risk a sure thing by asking him to get all innovative and daring?
For me anyway, the Brosnan films are a mish-mash of conflicting styles and tones that never add up to much of anything, because they try to cherry-pick the highlights of all that went before, even when those earlier entries were to some degree incompatible with each other. I agree with hilly that "Brosnan ticked off all the boxes" but for me, that's what's wrong with them; they feel built by committee in a soulless exercise: One-liners? Check. Explosions? Check. Car chase? Check. Good-looking guy in a tux? Check. Congratulations, gentlemen, we have a Bond film, let the profits commence.
And for the record, I'm not a "revisionist" critic of Brosnan: I disliked his entries when they were new. Even GE got a "meh" from me. As I left the theater after DAD, I decided that after five strikes (I wasn't big on LTK, either) even Bond was "out," so I wandered away from Bond fandom until CR, which I greatly enjoyed probably *because* I no longer cared about Bond, and so had no demands that it be this or that. But I wasn't big on QoS or SF, either, so I'm pretty sure I've just entered into "stay off my lawn" crusty curmudgeon territory. But the point is, as complainers go, I'm no come-lately. :-)