Developer time/team resources, and the "appropriateness" of contributing code back.
Meaning that, if we make modifications to an OSS project, sometimes the changes aren't necessarily appropriate to contribute back to the project. This may be because of IP rights, but actually, the most common reason is that we simply don't anticipate that other people would require such specific modifications to the software in the way that we've made it. So generally speaking, such patches don't make sense to send back to the team developing the OSS project.
In other cases, these changes could be sent as a patch to the OSS project developers, but this would require cleaning up/reformatting the code, separating out private company data from the patch, etc. Usually, if we're using OSS software in the beginning, we don't care about such things, because most OSS software is somewhat dirty in terms of code quality anyways (ie, no test cases, coding standards, documentation, etc.). Therefore, the time required to clean up our dirty fixes to already dirty code is usually more time than we want to spend for the altruism factor here.
That said, I have worked at companies that did contribute back to OSS projects when necessary, and those that did not made monetary contributions to some OSS projects or distributions.
