Sketch’s file format changed from a package in 3.1 because it caused a whole manner of issues when it came to sync services like Dropbox and Google Cloud. It was unfeasible to continue with this so we needed to make the change.
I'm pretty sure the iWork apps have been following this same trend (from packages to binaries). Just the problems caused by email apps & web servers seeing a folder instead of a file have been sufficient to drive developers away from it.
Sketch’s file format changed from a package in 3.1 because it caused a whole manner of issues when it came to sync services like Dropbox and Google Cloud. It was unfeasible to continue with this so we needed to make the change.