Not wishing to put words in the mouths of the creators, but there are a few reasons why one might target 2.7 rather than 3.x:

1. 2.7 is a stable target. There will be no further features added to the 2.x series.

2. 2.7 is a known target. Other experiments like PyPy have already tried many paths to improve 2.7, so what doesn't work is reasonably well known.

3. 2.7 is a widely deployed target. This is less important from an "adoption" point of view, more so that there is a lot of software written that targets 2.x, meaning there's more software to test with.

4. Finally, I suspect much of Dropbox's codebase is 2.7[1], and since this is an effort to improve things for Dropbox, it makes sense to spend the initial efforts there :)

[1] checking the version of Python bundled with the Dropbox client confirms that it, at least, is 2.7

  /Applications/Dropbox.app/Contents/MacOS/python --version
  Python 2.7.3