How does Database Snapshots Work (MySQL)?
In this specific case, it's because the sequential restore of a blob of data is faster than the traversal in order that MySQL does it in, and because MySQL could be CPU bottlenecked during some parts of the operation. Additionally, snapshot restores in AWS may not be constrained by the storage IOPS you are paying for.

In general, restoring a snapshot can be incredibly quick - with ZFS a rollback to a snapshot doesn't require restoring any data at all, it involves only marking newer data blocks as discarded. This takes milliseconds to seconds, rather than minutes.