That's a big part of it. And people can have so many different set ups now too. And there isn't time/funding to redo the mix for them all. There was a good article that covered some of the various reasons, I can't find it but some others...
The article I'm thinking of also mentioned mics changing, and actors not having to speak directly into it to get anything. So it opens up far more realistic acting, but makes capturing/mixing that dialogue more difficult.