Do I think LTK underperformed because U.S. audiences weren't ready for Timothy Dalton? Absolutely. Do I think that has anything to do with Dalton himself? Not a bit. Dalton went from "Yeah, he was a good Bond." On first viewing of his films to usurping Connery in the top spot after reading the books and reviewing his films.
What hampered Dalton most was events taking place after Roger Moore stepped down. "Remington Steele" was the "it" T.V. Show in the U.S. around that time. When a poll was done in the U.S. as to who should replace Roger Moore, Pierce Brosnan won by a landslide, and why not? RS has many of the elements of the Roger Moore characterization of Bond that were well-liked.
But Broccoli sought out Dalton first, and if history is to be believed, Cubby never sought out an actor for Bond more than Timothy Dalton from 1969 on. Dalton, however, was wrapped up in filming "Brenda Starr", and said no. Cubby then tested, and cast Brosnan, and of course everyone in the US was excited. But NBC did what they did, and Brosnan was out, and the time it took to settle that score gave Dalton the time he needed to finish his work on "Brenda Starr" (And if I remember correctly, he finished filming on a Saturday, and was at Gibraltar to film the PTS of TLD on that Monday.
How did this affect Dalton? Because I think, right from the start, he was unfairly maligned because he was considered the '2nd Choice', and everyone was hot for Brosnan at the time. I'm not saying, mind you, that I think Dalton's outings (Well, outing, because they'd have never written LTK if Brosnan was in the role) wouldn't have worked with Brosnan, but Dalton seemed at the time, (And still to this day) to be considered by a lot of people as the 2nd, lesser choice, even though, IMO, he's much better than Brosnan (Who I like just fine) and he was never the 2nd choice in the first place. We have the internet now, and can find that out. But not in 1986-87.
Couple that with the fact that Dalton went the polar opposite way of Roger Moore, which while lauded by some, was considered blasphemy by others. He's too serious, he's not funny, I hear that over and over, yet I think he's very funny in TLD and has funny moments in LTK as well. The difference is the delivery. Dalton can make being "pissed off" funny. I still chuckle every time at the scene in TLD when he tells Kara they're not getting the cello, then in the next scene, there he is, moodily waiting outside for her. The only thing that would have made it more humorous would have been to have him angrily smoking a cigarette as well. And the eyeroll when he's trying to get her to drive into the plane is priceless.
Dalton's humor is there, it's there in a great many spots. It's just that Dalton does the joke and moves on, and doesn't insist on winking at the camera once the joke is done. Tonally, there's less in LTK, but there has to be. I will always believe that Bond revenging Felix is only a small part of the story. I think what sets Bond in motion most of all is the fact that Leiter lost his wife on his wedding day. It's the same thing he went through, and the scene where Della throws him the garter is there for a reason. To remind the audience that Bond was married once, and set in motion that what probably sets him off after Sanchez most of all is the death of Della bringing those demons back to the forefront. Later confirmed in the scene with M. "Spare me this sentimental rubbish. He knew the risks." "Did his WIFE?"
That brief, but awkward pause is there for a reason as well.
I think both Dalton's outings are brilliant, and I applaud him as an actor. I don't think even Sean Connery could have played Bond as strongly as Dalton did. And even Roger Moore's second outing faltered somewhat at the box office, despite having Christopher Lee. It's a shame Dalton didn't get his own "The Spy Who Loved Me", to answer the question once and for all.
Why did it falter in the U.S.? There's no direct answer. It's a variety of issues. One of the things that I do think really hurt it was an 11th hour change of the film's title. Originally marketed as "Licence Revoked" it was changed to "Licence to Kill" at the last minute because people either didn't know what "revoked" meant, or they felt that it sounded like Bond was losing his driving priviledges. So they not only wasted money on the advertising materials for that, they then had to rush for advertising with the new title, and so yes, it was poorly marketed. That didn't help at all.
It's a crying shame Dalton only got 2 films. He had so much to offer the role, and was a brilliant actor. But at least we got the two films we did.
Here are some of the concept posters with that title, several of them a damn sight better than the generic posters they rushed to create.