Great film - Why wasn't this film appreciated in its day?

Well, let's see: Because it isn't much good? I bet that's it. Because the characters are manipulated and do things they would never do (or at least have shown no tendencies toward doing) to serve the plot? Because it is hackneyed and cliche-ridden? The girl is an airline stewardess, for cryin' out loud. This movie was derided in 1964 and is derided today, for excellent reasons. And, as we all know, it wasn't the last clunker shot by Frankie boy, though he also had some good movies ahead. The only reason people go see this movie today is the director's name. I saw it this afternoon at Film Forum in New York City and as it ended a few people applauded. Others in the audience then laughed and the applause died away in embarrassment.

Rewatched the film yesterday evening. It was yet again a marvelous experience. How the film jumps right into action, following an as of yet unknown man, racing around town to catch his plane. In this opening sequence Truffaut has established the film's primary motifs, time and chance, introduced the film's 'hero' and his situation in life and has you already glued to the screen. The entire scene seems also inspired by Hitchcock, with the Psycho like music playing during the driving scenes and close ups of little details, like the changing of gears, creating a great sense of urgency.
I absolutely love the scene in the plane, where Pierre spots Nicole for the first time. Both trying to catch glimpses of each other without the other noticing. How he watches how she changes her shoes behind the curtain. Truffaut really knew how to portray human interaction with the utmost honesty.
What is also very much Truffaut is how the man is the weaker person. Both his wife and Nicole are both very strong women, both looking for affection, but both much stronger than Pierre. In the end, Pierre realizes he has gained nothing from his affair, in fact he lost his wife, but what he doesn't know is that he's already out of time, due to the coincidence of his wife finding his photo order. The build up to the climactic final scene is yet again very Hitchcockian and just great cinema.
You are right to state that this film seems less New Wave and perhaps that's part of the reason for this film's critical 'failure'. Critics were expecting Truffaut, a leader of the New Wave, to bring them another film in that style, but they got something more old fashioned.

You said, "I love you," I said, "Wait"
I was going to say, "Take me," you said, "Go away"

They didn`t like it because they want warm, fuzzy adultery movies like Bridges of Madison County and Brief Encounter.

Besides, he must have been nuts to go looking for it elsewhere when he has a total babe at home who looks as good as Nelly Benedetti did in her early 40s.

By the way, how`s your mom, Ed?

People had come to expect youthful and exuberant films from Truffaut and this one was melancholy and more like a Rohmer film in its milieu and themes. I think there are some remarkable scenes in it, specifically after he leaves the lecture and has to watch at a distance as the woman he loves is harassed. It's like a nightmare; he wants to intervene but is rooted in place. Is it propriety, is it fear of being exposed, or is he simply a man who is incapable of acting? Also, the scene with the wife reacting to the womanizer after getting the photos developed. You expect her to just yell at him and get a "Crazy b****!" type response out of him, but one feels that this woman may have genuinely shaken him in his confidence.