They fell out of favour because the UCI doesn’t allow them in road TTs. Now that doesn’t matter to pure triathlon bikes (as they’re not governed by the UCI) bu

They fell out of favour because the UCI doesn’t allow them in road TTs. Now that doesn’t matter to pure triathlon bikes (as they’re not governed by the UCI) but it matters to the manufacturers accessing a much smaller sub-market. It also matters to the customers who might want to also race UCI events or resell a bike. Essentially double diamond designs are competitive enough now to make non-double diamond unviable.