I'm not sure that this is inherent to Stirling engines, or just the past engineering implementations. For one thing, unlike internal combustion engines, there is no need to contain an explosion (otto cycle) or flame front (diesel) so the pistons and cylinder walls can be made of lighter materials and don't need lubricant filters. In fact self lubricating graphite pistons are often used modern Stirling engines. Secondly a higher efficiency implies that more power is extracted from the fuel which indicates that the combined fuel tank and engine can be smaller and that more power will be put to doing work. Lastly, a Stirling engine usually falls short in torque, and that can be compensated for with a hybrid system that utilizes electric motors for instantaneous changes speed, where added torque is required.