On the GDScript introductory page (http://docs.godotengine.org/en/latest/reference/gdscript.htm...), they give the explanation of the shortcomings of other languages:

- Godot embeds scripts in nodes, most languages are not designed with this in mind. - Godot uses several built-in data types for 2D and 3D math, script languages do not provide this, and binding them is inefficient. - Godot uses threads heavily for lifting and initializing data from the net or disk, script interpreters for common languages are not friendly to this. - Godot already has a memory management model for resources, most script languages provide their own, which resulted in duplicate effort and bugs. - Binding code is always messy and results in several failure points, unexpected bugs and generally low maintainability.

The gave a shot at LUA, and it didn't work out well.