The short answer is that the appimages can easily bundle incompatibilities with current desktop libraries. This is a major issue with appimage (and what makes it non-universal) especially when built on an old distribution. But it needs to be as compiling against glibc usually means you can run it only on newer versions, but not older.
It can be loaded via:
LD_PRELOAD=/usr/lib64/libX11.so.6 ./FreeCAD_0.18-16146-Linux-Conda_Py3Qt5_glibc2.12-x86_64.AppImage