I remember distinctly because while I liked BSL, I never understood why I couldn't use AGPL as fallback (which would be much better suited to my use case).
EDIT: It seems I was wrong:
> To specify as the Change License the GPL Version 2.0 or any later version, or a license that is compatible with GPL Version 2.0 or a later version, where “compatible” means that software provided under the Change License can be included in a program with software provided under GPL Version 2.0 or a later version. Licensor may specify additional Change Licenses without limitation.
So Apache is OK, because such software can be incorporated into GPL app. AGPL is not, because it places additional restrictions.