I would be willing to bet an economic externality such as the cost and availability of Starfighters and crews. Especially for what appears to be a US based company.
The Air Force and NASA used the planes for high-altitude test missions, and so there is some comfort in outsourcing those missions to the company. I've always been under the impression that most of their business was government stuff, but I could be all wrong about that.
Even the MiG 29 would get you marginally better Mach numbers (2.2 vs 2.0) and better payload. Plus they were produced in similar numbers as the F104.