Professional baseball players would not like to chase down a fly ball, only to twist an ankle on some overly-trodden grass. Imagine the wild hop a baseball would take if the surface at Miller park was trampled flat by 50,000 fans. Some sports venues are routinely used for non-sports events. Soldier Field comes to mind. Jimmy Buffett played there a few years back, as did Pearl Jam and The Grateful Dead. I suppose that it's a bit easier to fix a football field when you have months to get it ready for the NFL season. You could have a concert in July, and still have nearly two months to get the grass back in shape.
For a "baseball only" venue there is the problem of trying to have a concert, then most likely a baseball game within days of each other. I remember the plywood covered outfield at County Stadium for Pink Floyd. 60,000 people crushed the field flat, but it didn't matter, the Brewer season was over.
Miller park keeps the roof open during the winter months. The snow can pile high in that place for sure. It's actually good for the natural grass to have a blanket of snow. Will that venue ever host a concert? They did just that during the 105th Harley Davidson Rally. Kid Rock headlined a two-night event there. The Miller Park Tickets you needed to get into the event allowed access to the parking lot only. The event was held outside of the ballpark.
As the years roll on, we might see a show inside the ballpark, but I don't see it happening during the baseball season. Perhaps there would be a show in late fall, with the roof closed, after the Brewers season is over. As a venue, Miller Park is not hurting for revenue. That may never happen.