Making an antimatter reactor with indestructible elements (like PCLN and FILT) is too easy, so people don't find this reactor interesting enough.
Also the engine - mostly CRAY (NEUT), wires and reactor, without interesting circuits and reactions. Since the basic part of engine is just CRAY, and antimatter is used only to create the SPRK, instead of an antimatter reactor, there could be, for example, a GLOW/PHOT reactor, and nothing would have changed.
The design is good, but not good enough to put upvote just for it.