Fuel enrichment was originally an extremely expensive service: Part of the advantage to the CANDU and RBMK designs was they didn't need it (CANDU still doesn't, modern RBMK does). Back then, uranium enrichment was primarily accomplished through an extremely energy-intensive process called 'gaseous diffusion' that required large facilities and infrastructure. Then the gas centrifuges arrived, and cut enrichment costs by an order of magnitude.

The last US gaseous diffusion plant was shut down in 2013, but it was uneconomic far prior (IIRC, it was kept around for DoE weapons purposes since that uranium can't be civilian-procured). Modern enrichment is relatively cheap now, which is part of the reason the nuclear industry is interested in boosting enrichment rates (which would have been prohibitively expensive originally).