Except for the "Clockwork Angels" tour when they had a string ensemble, Rush certainly was always only a trio playing live. To my knowledge, so was Cream and ZZ Top. And the Jimi Hendrix Experience definitely was.
Maybe it has to do with their vintage. More "modern" bands perhaps are under more pressure to replicate the album sound, which they can't do by themselves? Replicating an album doesn't give much room for improvisation, like Cream or Hendrix would be doing. And it has to be much less effort to have all the backup players. In Rush, for example, when they performed a song with keyboards, Geddy Lee would play keyboards with his hands, bass with his feet, and sing. Not much of that going on anymore...
It's funny that you mention this, there are a lot of bands that have added a lot of players in the shadows, not just trios. The Eagles, Fleetwood Mac, Pink Floyd, and so on. The "American Idolization" of live music.
walrus