Snaps evolved from click packages that were developed for the Ubuntu phone. The design dates back to 2013: https://lists.ubuntu.com/archives/ubuntu-devel/2013-May/0370...

Flatpaks, according to Wikipedia, were first released in 2015. I don't about the current situation, but initially a bunch of features that are considered necessary for snaps were out of scope of the Flatpak project, such as IoT support.

So it wasn't so much that Ubuntu selected snaps over Flatpak. Flatpaks came along later, and Ubuntu had already invested considerable resources developing snaps.