My guess would be that there is no shortage of these kinds of locomotives on the secondary market. In other words, the market is soft. And, with some exceptions, the prices on these engines on the secondary market are a bargain in comparison.
Now factor in the rising costs to produce new product and I think there within is the answer. In the case of Williams, there were a lot of deep blowouts to move existing inventory. So from their standpoint, I can imagine they would want to wait for sufficient market demand.
With Lionel, it seems pretty clear that they are putting their emphasis on the LionChief products. They're trying to reach a new generation with the starter sets. And with the LionChief Plus locos - which by all accounts seem to be well received - those seem to be aimed at conventional operators to make the next step toward command operation and features, without having an entire command system.
A few years ago when Lionel and Williams had more of the sort of products you mention Greg, a frequent comment made was along the lines of "Wow, look at these prices! Guess I'll have to stick with the used market."
A point often gets made, how can Mendard's offer products for so much less than the established train companies? I think right there is part of the answer: The other companies have entire product lines, catalogs, service departments, distributor networks etc., which all adds the cost of a product.
And not the least of which is the advance financial investment required in R&D and tooling to bring any brand new product to market. Unfortunately these costs do get spread out all over a given company's product line.
I think it also might be akin to the situation with 027 tubular track from Lionel. A couple years ago Lionel announced that it would be made only on a "demand" basis. When there was enough distributor/dealer demand for the product, it only then would be made.