Why is KSP more popular? Because it's a sandbox game, whereas Orbiter is a simulator. Most people prefer the freedom provided in a sandbox game.
KSP started as a sandbox and got popular that way. I still prefer sandbox play to the career mode. I think a large part of KSP's success is the other issue that's been touched on in this thread: all add-ons for KSP are required by the KSP license to publish full source code, the add-on community there has evolved around a culture of permissive licensing, and add-ons are trivial to install and uninstall. KSP, like Skyrim and Minecraft, owes a significant portion of its popularity to the add-on makers. And the add-on culture there--unlike here--is open, robust, replete with tools, guides and tons of source to learn from, and continuously churning.