Although the MAVLink spec defines a standard communication protocol to communicate with a vehicle. There are many aspects of that spec that at up for interpretation by the firmware developers. Because of this there are many cases where communication with a vehicle running one firmware must be slightly different to communication with a vehicle running a different firmware in order to accomplish the same task. Also each firmware may implement a subset of the MAVLink command set.
Another major issue is that the MAVLink spec does not cover vehicle configuration or a common parameter set. Due to this all code which relates to vehicle config ends up being firmware specific. Also any code which must refer to a specific parameter is also firmware specific.
Given all of these differences between firmware implementations it can be quite tricky to create a single ground station application that can support each without having the codebase degrade into a massive pile of if/then/else statements peppered everywhere based on the firmware the vehicle is using.
QGC uses a plugin architecture to isolate the firmware specific code from the code which is generic to all firmwares.