I found myself watching with an eye to the technical side of things. I'd watched Young Frankenstein recently with the same viewpoint, and there were some interesting contrasts.
Oliver! had very few costume changes--most cast members wore the same costume throughout the show, or changed only once or twice. But I found myself noting that some songs went on for a very long time--after doing all the verses of "Consider Yourself", for example, they repeated the whole song from the beginning. I found myself wondering what purpose was served by dragging out the song like that, and I speculated that it was buying time to reconfigure the side of the set that was currently out of sight for the next time it revolved into view.
Young Frankenstein did not do such significant set changes, but it did far more elaborate costume changes, and far more quickly. YF would take the ensemble offstage, do a song that was short enough that I didn't notice the padding, and bring them back onstage with a costume change that included changing stockings and changing wigs.
So, I infer that one of the following things is true:
• Changing costumes is much faster than changing sets.
• Theatrical technology has improved sufficiently in 50 years that YF is willing to do costume-changing feats quickly while Oliver! had to buy time for feats of comparable difficulty.
• Instead of a difference of tech level, it's a difference of budget; YF invested in quick-change skills, but Oliver! invested in songs catchy enough that they could be repeated.
• I'm wrong about the purpose of the drawn-out songs. Perhaps the creators just feel that it's terribly important that the audience see how Oliver is welcomed to the criminal fraternity.