A corp has more flexibility in what it's allowed to do, compared to a US nonprofit. Maybe similar reasons why Mozilla has the Mozilla Corporation in addition to the nonprofit Mozilla Foundation.
The non-profit Mozilla Foundation had many rules about what we could and could not do. The new Corp gives us more freedom, legally.
...unless there are plans for profit and charging a subscription or some such.
Thunderbird will now be under MZLA Corporation, which is owned by Mozilla Foundation.
Why not put it pack in Mozilla Corporation? Because Mozilla Corporation has tried to jettison it a few times for not aligning with their goals, I guess?