Because it is classed as an air pistol, you just have to give a name and address - you can only buy them over the counter - there is no mail order option due to the VCR act (violent crime reduction).
Airsoft guns on the other hand are not classed as air pistols, and are therefore classed as realistic imitation firearms (rif). To buy a rif you must be an active member of an airsoft group and have attended at least three sessions.
The law is daft really - this is more powerful than the airsoft equivalent - looks equally as realistic, but one is classed as a realistic imitation firearm and needs a legitimate reason to own but the other doesn't