Money ?
I suspect that the WAM diesel is being developed on a very tight budget.
I was in the position to offer an airframe to put the motor into and to do the work to get an STC from the CAA however all that WAM wanted to do was sell us an engine not to lend an engine untill the engine/airframe combination was ready for the market despite the fact that we could see the market for 1500 units.
I wish WAM the best of luck and think that the engine could do very well as it is a well engineered product but I suspect that it will fall by the wayside due to lack of finance.
However I would be very pleased to be proven totaly wrong about this.