1. Both LibreOffice and Thunderbird are desktop implementations of applications for which most of the recent momentum has been behind web-based implementations (Google docs, GMail, Office 365, etc.). So TDF is used to the desktop vs. web dynamic.
2. Both LibreOffice and Thunderbird has large installations in business/government environments.
3. Both LibreOffice and Thunderbird started out being governed by different organizations and would be adopted by TDF (slightly different situation for LibreOffice which was a fork, though Thunderbird will be forking off its shared base with Firefox). So TDF is used to adopting legacy code and continuing to evolve it.
That said, I hope the TDF scrutinizes Thunderbird and only takes it on if it is confident that Thunderbird fits into its overall mission and infrastructure. In recent times I have liked what I have seen from TDF and been somewhat disappointed by Mozilla, so I wouldn't Thunderbird to hamper LibreOffice's momentum.