And Gollum interacting and blending into his environment better than Yoda is very subjective. There are plenty of shots in The Two Towers were Weta didn't even get remotely close with blending in Gollum to the environment.
As far as using stand-in's compared to using Andy, that has to do with design. Most of the designs of the aliens in the Prequels are far more alien than the design of Gollum. Technically, Jackson could've gotten an extremely skinny person, apply plenty of make-up, and it would've worked just as good as the CG version of Gollum. It's almost impossible to find someone that skinny to match what they did with Gollum but it is possible.