There are multiple reasons: 1. it already supports more formats [1] than atool. 2. It's written in Python and easily extendable by anyone interested supporting their own formats. [2] 3. It's probably faster because of hyperscan. 4. More reasons :) [3]

[1]: https://unblob.org/formats/

[2]: https://unblob.org/development/#writing-handlers

[3]: https://unblob.org/#why-unblob

`unblob` is closer to `binwalk` than it is to `aunpack`

See: https://github.com/ReFirmLabs/binwalk