I dont really think its about one manufacturer or another 'calling dibs' on a particular loco or class, but models need to sell in large enough quantities to pay back the investment required in designing and producing such beautiful models. Some popular or common types will have enough variations to support more than one manufacturer producing models and everyone makes money.
But should models not sell in enough quantities then it could be disastrous, particularly for the smaller manufacturers and DJM as an example is really only starting up in comparison to the more established producers. If more than one manufacturer is competing for the same sales then both could lose out.
Hands up who wants to gamble their house on how many of a particular new model is going to sell well.