A pretty simple model that accounts for the data is that Moore's law, and many other exponential growth examples, require ever larger capital expenditures. This worked for Moore's law because at ever step of improvement the devices produced were highly economically valuable. For fusion, on the other hand, you can have an exponentially improving triple product, but it has zero economic value until you cross the net-positive threshold. That basically means that the exponentially increasing development funding needs to be provided by the government, philanthropy, or some other non-profit source. If you're exponentially improving, with exponential costs, and you hit the ceiling of what the government and philanthropists are willing to provide, your progress can come to an abrupt halt without it necessarily meaning the basic exponential engineering curve you were following stops.