"The primary obstacle to replacing uranium with thorium is the immense "lock-in" effect created by the global nuclear industry’s multi-trillion-dollar investment in the uranium-based light-water reactor (LWR) infrastructure. Because the existing fleet of over 400 reactors was designed specifically for uranium fuel, transitioning to thorium would require not only the development of entirely new reactor technologies—such as Molten Salt Reactors—but also the creation of a completely new, large-scale industrial supply chain for thorium fuel fabrication and chemical processing. This economic and logistical inertia is compounded by the fact that the uranium fuel cycle is a mature, globally regulated system, whereas thorium technology lacks the commercial-scale demonstration and regulatory framework necessary to compete with the established, highly optimized uranium industry."
Source: 'Large, Stanford. Thorium as a Nuclear Fuel Source.'