Because he has Avalon, I presume.
You can't regenerate if there is nothing left. You can't come back from being a smear on the ground, or being vaporized.
Except nothing can even hurt him. Dude, Avalon can tank ea even at full power. Puny Types can do nothing to Satoshi.
Also, its a good thing this is omake. Its both confusing and the nasuverse continuity is being fucked up for the sake of this story working. Lantz, you pull in Nasuverse characters and then say its similar to their universe but all the important bits are different. Have you considered explaining any of this before throwing out such a weird situation? At this point, its Nasuverse in name only.