Perhaps because pethidine has additional dopaminergic effects, ren. Plus its metabolite, norpethidine is toxic,in an excitatory, proconvulsant manner.
Still, its in my top three opioids I'd want to try, along with dipipanone and ketobemidone. And as a runner-up, levorphanol, which has additional NMDA antagonist (dissociative, hopefully) properties. Dipipanone sounds like it might be especially appealing, as an analog of methadone, another opioid with (in methadones case albeit moderately and only becoming apparent at higher doses, such as require a pre-existing tolerance to tolerate sufficient levels for those secondary effects to become apparent)
Dipipanone is very seldom prescribed here anymore, but has one hell of a reputation, enough to make it worth my considering for a future project once I get the current lot off my plate, so to speak.