It's curious that a mention of Benny Goodman showed up in a different branch of this thread, because I was going to answer Albert's original question ("In between the early jazz ensemble style and the contemporary formula of a sequence of solos, things changed ... When did the change come about, what artists were mostly responsible for the change, and what are the earliest examples of arrangements consisting of sequences of solos?") by mentioning the Benny Goodman small group sides from the 1930's, specifically the great sextet with Charlie Christian. In my opinion, Goodman's small group efforts represent the first consistent example of the contemporary jazz formula of 'head in / solos / head out', with very little group ensemble work.
Goodman probably best exemplifies the trend that began in the 1930's with the advent of the "band within a band" -- e.g. The Goodman Trio and Quartet, Artie Shaw's Grammercy Five, Tommy Dorsey's Clambake Seven, Bob Crosby's Bobcats, etc. Written arrangements and ensemble playing were retained by the full band, but 'head-solos-head' became the format used by the small jazz groups. Of course there were exceptions (Raymond Scott's small group being the best one) but the general tendency that evolved during the Big Band Era was that the bigger groups used arrangements, while the smaller groups concentrated on solos.
From here on out I can only speculate, but my guess would be that top soloists and improvisers enjoyed soloing more than playing ensemble parts, and with the advent of bebop and its heavy emphasis on soloing, along with the commercial demise of large dance bands, the small group 'head-solos-head' format became the standard for jazz. About the only time ensemble playing turns up in mainstream jazz these days is if the band is playing a jazz tune written by a musician (as opposed to a standard written by a songwriter) and the tune itself includes ensemble parts in the head.
Perhaps something else to consider is the size of bands in the 1920's. There were few "big bands" during that time. Most dance bands were 5 - 10 piece outfits. Size-wise, they may visually resemble today's mainstream jazz groups (two or three horns, piano, bass, guitar, drums) but functionally they operated very differently. Most dance bands of the 20's consisted of musicians who were not skilled improvisers, and relied on written arrangements almost exclusively. Audiences expected (and still expect!) to dance to recognizable tunes. The melody was in demand, in other words. In that regard "Singin' The Blues" is a remarkable record, simply because the improvising starts without the band ever stating the melody. In fact, we don't hear the melody until the out chorus. But today, mainstream jazz groups play primarily for listening, not dancing. Audiences expect to hear improvisation, and are more interested in solos than in listening to the melodies of standards.
Off the top of my head, I can't think of another performance like "Singin' The Blues" (introduction, improvisation, melody only shows up in the out chorus) from that era. Jazz historians tend to point to Bird's record of "Embraceable You" as a landmark, because he begins improvising without stating the melody, but Bix and Tram blazed the same trail twenty years earlier.