Memory / performance limitations most likely. Though I don't know why they won't / haven't provided an option to allow the ability of Kit style sample arrangments, at the expense of Parts... the reality is Parts confuse some, are useless to others, and good for some.
I would imagine traditional Kits, with a lot more limitations then a Part, would be an easier concept/usage for most people to grasp.
Delay and Reverb are the two most processor intensive effects, no? Which basically translates to the OT doesn't have enough horsepower in the engine to run a Delay/Reverb effects chain, definitely not across eight track simultenously.
Elektron have stated multiple times that this (Midi Scenes) are not possible and won't be implemented.
Really they are scared about either the amount of processing the OT can handle, or the amount of midi data potentially being sent and causing midi chokes, hanging notes etc on the receiveng gear and screwing up your midi stream, still, where theres a will theres a way.