My reason is that I want to use the best available tool I can get my hands on, so that I can do right by the people that use my software (provide them with the new features and support they want, as quickly and cost-effectively as is possible).
GPL advocates say that doing right by my users amounts to providing them with 100% of the source code of the applications that I and any collaborators write.
Most of my users are not technical, and could not care less about the source code behind the apps they use. They just want something that's easy to use and reliable.
Some people see this very differently than I do, but my take is that being moral means doing right by and for yourself first, where "yourself" includes concern for the specific people and things that are important in your life. For people you don't know personally, the right attitude is benevolence (unless and until a particular person gives you a good reason to avoid them.)
Maybe some people say that I could live by that standard with the GPL, but my own judgement is that I (as well as the people and things that I care about) am far better off without being encumbered by it in the slightest.