For me, it's primarily the cost and following on from that, the availability of parts. Down here in Australia, parts for an FE are generally about 20-30% more expensive than what you pay in the US, factoring in shipping, exchange rates and customs fees.
I have 2 FE powered cars, a 63 Galaxie that is mainly a street car, and a 64 Fairlane drag car. Both cars have the engine bay that will accommodate a SOHC engine, so that's not an issue. However, the SOHC in the Galaxie would purely be for pose value, as the new 462 cube 6V engine in it currently makes way more power than I can use on the street. So if I built a SOHC engine, it would be a full race engine for my drag car. And that is where I find the cost becomes too expensive, even compared to a full race wedge head FE.
I did partially venture into getting parts together to put a SOHC together and decided to try using our local parts source. However, the parts I did receive were not to the quality I want, and I am still awaiting an outcome of the cancellation of the remainder of my order. I don't really want to air my experiences here, but mention it only as another reason why I am no longer going down that path.
I would love to own and run a SOHC in either of my cars, as Jay said, they are the pinnacle of FE power. But reality of the shallow depths of my pockets, complexity of assembling and maintaining the engine, and availability of parts are the reasons I will probably never own one.