I know Sondheim was very much in favor of the switch of "Cool" and "Krupke," but I was curious what the argument was for the way it was originally done, and how the rest of the creators felt about it. So I did some online research -- can't say it's any sort of official sources, as most are from secondhand sources saying what they've said, but it's an interesting, and somewhat contradicting picture.
A Sondheim/Frank Rich discussion had this quote:
"And speaking of why musicals are they way they are, [Sondheim's] anecdote about wanting to flip-flop West Side Story's Act 1 "Gee, Office Krupke" for Act 2's "Cool," because he couldn't' see the logic in the gangs having killed two people and then breaking into comedy, shed light. Robbins agreed with Sondheim's idea. Leonard Bernstein agreed. Arthur Laurents agreed. "Cool" made better post-stabbing sense for the Jets and Sharks. But, Robbins said he couldn't make the switch, because the choreography for "Cool" needed the whole stage and the "Krupke" moment occupied only the very front stage zone, while the set was changed behind the dancers."
This technical aspect has been mentioned in several places, so if not being an ultimate reason, it seemed to play a part.
But the one thing that isn't consistent is how Laurents felt about this -- most places I checked seem to indicate he did not agree with Sondheim. For instance,
"and while it's terribly self-serving, Laurents' ORIGINAL STORY claims Krupke was Laurents' idea alone - SPECIFICALLY - to offer a comedy relief number/10 o'clock number between the two bedroom scenes. Please note there was (in the gospel according to Laurents) no debate - at all - about moving Krupke about or having "Cool" serve as a second act production number. In fact, Bernstein, Robbins and Sondheim were originally against the idea of having a scene/number in that place at all."
This seems to concur with the thesis already put forward on this thread about Laurents' ego...
NPR further posits that it came down to Sondheim vs. Laurents...
"They didn't [switch the places of the songs] because Laurents was insistent: "I thought that the tension in the second act needed relief," he says. "And they thought I was being vulgar and inserting musical comedy. So, I got very high-falutin' and talked about Shakespeare and the Porter scenes and his clowns. It was me taking one position, and I had to convince the other three that it was the right position. And I did."
So there's some interesting different ideas, stories, and all that jazz. Make of it what you will. But, as much as Sondheim was for the switch in 1957 and loved how it was done in the movie, he wouldn't necessarily take the same stance for the revival. Supposedly:
"Sondheim has said that while the switch works well on film, it shouldn't be allowed in the stage version - because film and the stage are different media."
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli