Only two nations besides the US operated single seat Eagles, Japan and Israel. After that all F-15 production, including all advanced versions which includes all exports, have been based on the F-15E and consequently all are two-seat.
Since the F-15EX is essentially the F-15SA/QA with some US systems, it would be much too expensive to develop and produce a singe seat version. Remember, the F-15EX is substantially more expensive than the F-35 to buy, the savings come in the operating costs, so you want to keep that unit cost down. And although officially the EX has been announced as replacing the C and it's been stated it will operate as a single seater, there's a whole lot you can do beyond training, given the systems, displays and software are already there, when you put someone (can you say "WSO"?) in that rear seat... I'm sure someone will eventually "discover" this.