One issue seems to be that it wasn't seen as desirable to promote something that is not source-available under whatever license, which conflicts with the main idea of the TGPPL of keeping the software closed-source during the grace period. Another issue was ensuring the source becomes available after the grace period as intended. I may be wrong, though, as I haven't read even half of the extensive discussions on this topic.
OSI board meetings discussing TGPPL briefly: https://opensource.org/minutes20090205 https://opensource.org/minutes20090304 https://opensource.org/minutes20090401
Relevant mailing list threads:
Dec 2008 thread: https://lists.opensource.org/pipermail/license-review_lists....
Jan 2009 thread: https://lists.opensource.org/pipermail/license-review_lists....
Feb 2009 thread: https://lists.opensource.org/pipermail/license-review_lists....
Jul 2013 threads: https://lists.opensource.org/pipermail/license-discuss_lists... https://lists.opensource.org/pipermail/license-discuss_lists...
Dec 2013 message: https://lists.opensource.org/pipermail/license-discuss_lists...
Jul 2018 thread: https://lists.opensource.org/pipermail/license-discuss_lists...
You might also be slightly interested in this small Wikipedia page section relevant to the motivation for the TGPPL: https://en.wikipedia.org/wiki/Business_models_for_open-sourc...