That would be Apple themselves. They've declared that the maximum allowed OS for the A1176 is 10.7.5. There is apparently a hack to work around this and install a later OS.
However, with just 2GB of RAM, I would not advise it. OS X started requiring a whole lot more memory for itself somewhere around the 10.8/10.9 updates, to the point that it now consumes almost 2GB just for itself. Modern OS X really needs at least 4GB of RAM for comfort, and probably more than that for serious workloads. 🙁