MPE is, in effect, an extension to the pre-existing MIDI Mode 4 ("note per channel") used by guitar controllers since the '80s. It's not perfect, but it does provide reliable transport across any kind of transparent MIDI network (whether that's a five-pin DIN wire, USB, an OS MIDI stack, Bluetooth, a plug-in API etc.) and reliable recording and playback on existing systems.They could have. The thing is NRPN (and also SysEx). Is just too open ended for what they want to do. This entails that they have to add more on specification/standardization which means that they have to convince vendors to adopt the extra new spec. The more minimal changes they make, the more probable that companies will adopt.
(Yes, it gets broken by hosts that re-channelize i.e. Ableton, and yes it has some problems with edit/merge operations due to channel ID conflicts and non-atomicity.. however, NRPN-based implementations have the same problem x10).
Btw, the host needs to be responsible for sending the MPE configuration message, because you might re-load the project you recorded earlier without your Seaboard connected, or send the project to a colleague, and still want the tracks to play as MPE