On your broader question -- why not native 64-bit app -- I think the key reasons are 1) level of effort, 2) performance and 3) plugins:
1) Supporting 64-bits means a lot more than recompiling. It will need to be a separate development effort, probably doubling the resources required for the project.
2) 64-bit pointers are twice as large, and 64-bit programs take up more memory. Due to caching, this also means that the programs run slower.
3) All the plugin partners face similar problems of effort and performance. A 64-bit browser will necessarily ship with a lot fewer plugins than a 32-bit version.
Right now, a browser maker looking at a 64-bit version sees a lot of effort for a slower version with less functionality. And that just doesn't seem like a good bet.