Why APM Planner?

Sorry if I’m a little slow but I’ve read the post by Bill Bonney describing APM Planner and besides cross-platform, it looks and sounds just like Mission Planner to me.
I’m just trying to understand why develop another planner when we already have Mission Planner? Why not just invest the time in improving Mission Planner instead?
Thanks

Mission Planner does not run on Linux or OSX. Thats the reason. It looks and feels like MP so it easier to pick up and use. If you have any specific requests please let us know. Thx