As others have pointed out, Quantopian is trying to build a profitable, sustainable business, which means that our decisions about what to open-source need to balance the benefit to the community against the cost to Quantopian. If we give away so much of our work that we enable competitors to leapfrog us and put us out of business, that benefits neither us -- because our efforts to build a profitable business will have failed -- nor the open-source community -- because we will no longer be around to maintain and support the software we open-sourced.
Furthermore, when open-sourcing is done properly, it involves a great deal of time and effort, over and above just changing the status of the Github repository from "private" to "public." For an open-source project to be successful, it needs to be well-documented and well-supported. Also, though we obviously try to maintain high coding standards across our entire application code-base, we rely extensively on shared knowledge among our engineering team; to be perfectly frank, I think a lot of private code would require substantial clean-up before we would be comfortable making it public. :-/
There are certainly other pieces of our code-base in addition to Zipline that we're will eventually open-source, when we have the resources to do the necessary work. There are also pieces that we won't open-source, either because they're too valuable for us to give away, or because they're not valuable enough. By that, I mean that if a particular piece of our application wouldn't be of interest to a large enough group of people, then the benefit of open-sourcing it isn't worth the required effort. Our real-time market-feed ingester that produces the minute bars we feed into live algorithms is a probably a good example of that. The NxCore data feed is really expensive. Most of the people who can afford it are large enough trading shops that they're going to want to roll their own ingesters that are tailored to their environments.
On the specific question of "how much else Quantopian could (and does?) contribute back to the open-source community," we are active contributors to a number of open-source projects, most notably iPython, Jupyterhub, MongoDBProxy (though that project was started by a third party, it might not be an exaggeration to say that at this point the majority of code in it was written by Quantopian), and nose_gevented_multiprocess; we sponsor Boston Python meetups on a regular basis; we frequently give useful presentations at Boston Python meetups that are unrelated to Quantopian; we organize and sponsor regular, free Algorithmic Trading meetups all over the U.S. and occasionally in other countries as well; we organize and sponsor a regular, free lecture series about quantitative trading which uses the Quantopian platform as a teaching tool but is very much relevant even to people who do not use Quantopian; and we've open-sourced a number of other projects in addition to Zipline, including pyfolio, qgrid, pgcontents, coal-mine, qdb, and metautils.
I think, frankly, that we have every right to be proud of our efforts to support and grow the open-source and fintech communities. We have nothing to prove in this area, and our efforts to support and grow these communities will also continue to grow.