The Raspberry Pi has some unique features that are very powerful and easily accessible for a Hacking Kit. In particular, Pi is a joke and its components cost the price of a LEGO kit.
So, Raspberry being highly discreet, small, thin and easy to hide and of course most important, runs Kali Linux natively (without any adaptations or VMs).
It is very flexible and able to run a range of hacking tools, from badge cloners to scripts to cracking Wi-Fi networks.
Additionally, the low footprint and power consumption of the Raspberry Pi means that it is possible to run the device for a solid day or two on external battery pack USBs