Showboat was a megahit.
I just read a really good series of books about the history of musicals, decade by decade, and the author (Ethan Mordden) explains why Oklahoma! is so important: (he’s distinuishing between musical comedies like Anything Goes and musical plays)
In an earlier volume, he dismisses Show Boat as the first integrated musical because it wasn’t. What we see nowdays is a stripped down version–the “good parts” version, to use a Princess Bride-ism. Almost no-one alive has seen the “real” Show Boat–what we’ve seen is the “Let’s make Show Boat more like Oklahoma! version” that cropped up in the mid '40s.
For example, a tradition of pre R&H musicals was that you let the star do their “specialty” act–you’re a director who sees some woman who does a really spiffy bubble-dance act and so you tell the writers to write a spot for her into your new show. And so, in the middle of a zany story about a guy who must fulfill some weird condition to win his inhertance when the hero runs off, stage right after the heroine (who’s really the lost princess of Ruritania, unbeknownst to all) leaving the rest of the cast looking on. And then the specialty dancer says “Gee fellas, don’t be glum! Does anyone want to learn the new dance craze that’s sweeping the nation? Let me show it to you!” whereupon she goes into her dance while the scenery is prepared for the next act.
And the public was cool with this. To paraphrase the author "The audience at the time didn’t say “Who is this woman doing the bubble-dance and what the hell does she have to do with what’s going on?”, they thought “A bubble-dance! Cool!”. There were several (IIRC) in the original Showboat, the one that comes right to mind is “Kim’s Imatations” where, in the end when Kim is appearing in St. Louis on stage, the actress did her specialty act of impersonations (of Bea Lillie and Ethel Barrymore to name two) while singing a reprise of “Why Do I Love You”. This scene/song has since been replaced. There’s another scene where a redneck gets so caught up in the melodrama that the Showboat players are presenting and starts shooting at the actors. The original Cap’n Andy was apparently a Danny Kaye-esque comedian and the musical left the spot open for him to take over and do whatever he wanted (since the performers fled)–usually he’d finish the melodrama, taking all the parts himself.
Th’ thing with Oklahoma! is that everything is integrated–the songs, the dialogue, no “specialty acts” and most importantly–the dancing. The dancing actually moved the story forward and that was new. Take out the “Laurey Makes Up Her Mind” ballet and there’s a huge gaping hole in the plot–we’d have no idea really who Laurey is or why she’s so afraid of Jud. The dream-ballet got inside her head and let us get to know her hopes and fears.
By contrast, take out some of the dances in “Love Me Tonight” and you get a shorter movie–hell, take out the famous “Singin’ in the Rain” dance by Kelly from the movie and you end up with a shorter movie*. Oklahoma! integrated everything. It may not have been 100% successful (you need the “Will gets off the train” sequence, but the “Kansas City” dance number is there because, at best, it lends atmosphere. But it’s completely cuttable from a story standpoint.) but it tried. At least Will didn’t get on stage and do, say, impersonations 'cause it was his specialty actl
Fenris
*Not a better movie, mind you. Don’t get me wrong–Kelly’s “Singin’” dance is one of the best ever filmed. But it doesn’t tell us anything new about the character or his situation–he’s already fallen in love with Reynold’s character and we knew that several scenes back. The dance is there because Kelly was a genius and it made sense to give every possible second of screentime for him to show off his art. In other words, it’s a specialty act.