Because all or nothing forces someone to compromise. By default almost all vendors would pick to keep things proprietary. If going open source is the only possible option then some vendors will release the source and if they don't then hobbyists will write their own drivers since the work is very useful.

Wikipedia faced a similar issue when considering using https everywhere. Some countries were blocking certain pages on Wikipedia and https would break that so it was uncertain if they would just block the whole of Wikipedia now. As it turns out they mostly did not block the whole website and those selected pages became unblocked because the alternative was given by the all or nothing approach was too far.