According to joinmastodon.org/about :

Mastodon gGmbH is a non-profit from Germany that develops the Mastodon software.

[…]

Mastodon, Inc. is a non-profit entity in the United States that supports the growth and operational capabilities of Mastodon, including being able to receive tax-deductible U.S. donations and in-kind support.

Doesn’t seem like it was a move, just a different entity. Seems like there’s a bit more history to this if you want to look it up, for example the German GmbH lost its nonprofit status in 2024, strangely.