Because you have to add support for GCing of foreign structures from these libraries, or keep the interface low-level and force the user of the GCed language to

Because you have to add support for GCing of foreign structures from these libraries, or keep the interface low-level and force the user of the GCed language to manually manage these foreign objects. Adding support for GC can be difficult if not impossible, because GCs often arrange memory in special ways (different pools, etc.) whereas the foreign library probably just uses malloc.