Mechanical linkage for multiple direct drive external combustion engines that can operate at 0 rpm is far simpler (the drive axles are basically the crankshaft) than for a single internal combustion engine that must maintain a minimum engine rpm and has to be geared down to axles that are riding on a bogie that pivots separate from the deck of the engine.

To take advantage of the increased efficient of an internal combustion engine you'd need a lot of mechanical complexity unless you go hydraulic (solidly defeating most efficiency improvements) or electric.