As I'm on the Wrong Side of The Pond, my observations about US O Scale might be way out, but here's what I see:
The main Era of interest is still the Transition Era circa 1950s. Is that partly down to the average age of O Scalers? It's often true that we model what we recall from our youth, and those in O Scale tend to be older - so that's the Era the majority of current O Scalers remember & want to model.
Being older & likely retired, they will also have more 'disposable income' to spend on stuff. Younger modellers interested in, say, 1980s onwards, either aren't into O Scale anyway, or even if they are (like me) can't yet afford mega-bucks models, as they still have mortgages to pay & families to feed (like me!!)
And although prices are relative, for myself with a mortgage & family, Sunset prices ARE mega-bucks, so even if I didn't have to factor in Shipping cost to the UK plus 20% 'Value Added Tax', a Sunset SD40-2 would be way out of my pocket, so it's pointless for me to express interest. Maybe there's a lot of Stateside O Scalers in the same position, which gets mis-read as "no demand for modern diesels", rather than what it really is - "want modern diesels BUT can't afford them at that price".??
A question was asked a while ago on this Forum, after a discussion about the Santa Fe CF7 rebuilds - "would we buy it if Sunset made it?". My answer had to be a sad "No", because no matter how much I really want a CF7 (& I REALLY want a CF7!!!) I couldn't afford a Sunset model if they did it.
If there was more stuff out there similar to first-gen Weaver, & Red Caboose; i.e. plastic-bodied locos with can motor & chaindrive, for 2nd & 3rd generation diesels types, at an affordable price point a bit below current Atlas & MTH, then maybe it would attract those modellers of the 1980s-on scene, create the demand for more, and ensure 2-rail O Scale prospers, rather than dying off - literally - as the inevitable happens to those who remember the Transition Era, taking their demand for stock of that Era with them.
Just my thoughts....
