and a few others that come to mind (probably Austin, Grand Rapids area) still out-punch the big cities pound for pound by a good bit.

The well-known breweries actually in the city (Half Acre, Revolution, DryHop, Haymarket, maybe a few others) tend to be clustered where gentrification started occurring maybe 10 or 20 years ago, and land was relatively still cheap in those areas, at least compared to today and compared to what were the expensive areas back then. You can almost draw a semi-circle arc around where that was at the time and go right through the first line of brewery renaissance in the city. That line has now pushed out again into areas like Humboldt Park, Edgewater, Hermosa, almost into Belmont Cragin - would have been unthinkable ten years ago, but that's where the cheap land is now, and it's too expensive to do anything where the more established places are. I think that's why you don't see a cluster or a couple of clusters.