Given the above it might be reasonable to ask why the Harbour developers didn't elect to use the LGPL instead. The reason simply comes down to the requirements of the LGPL and what can be reasonably asked of anyone who wishes to make use of Harbour.
The LGPL requires that any binary distribution of the LGPLd code (in other words, an application compiled with Harbour and linked against the HVM and RTL) should be distributed in a method that allows the user to re-link the application against different and/or later copies of the code under the LGPL. This is an unreasonable and unworkable requirement as far as the Harbour developers are concerned.
For the most part it would be fine on platforms where an application can be linked against a dynamic version of the HVM and RTL, but any platform that doesn't support dynamic linking (PC/MS/DR/Open/Free-DOS for example) would be effectively shut out of Harbour usage. The Harbour developers consider this to be a reduction of freedom and this isn't acceptable for free software.