It also has a huge amount to do with the songs that Brian was writing. It would be hard to reproduce a lot of songs post-Summer Days with just five guys onstage.
Edited to add: There's a documentary in which this is discussed - I want to say it's "Brian Wilson, Songwriter"?
That makes perfect sense. You won't be able to recreate their music after the early days with just 2 guitars, a bass, and drum kit.
Thank you! I will check out the doc, too.