I don't get why many more FREDS aren't offered.

Fuel economy is the feature that I noted that FRED offered. The front engine benefits from better cooling from air when traveling. Thus the percent of the time the cooling fan in on is lower than a DP. The large cooling fan can consume 20% of the engine power. FREDs seem to be built of gasser chassis or modified gasser chassis and thus they may well be lighter than DPs helping their MPG. However, in balance I would prefer a DP as long as I can do so.