Not all unprocessed milk is good milk. Good quality raw milk is difficult to produce and very difficult to produce consistently. Using a highly variable raw material can make it very difficult for the cheesemaker to manage the cheese outcomes. Unless a cheesemaker can achieve some level of consistency, it is very difficult to be financially viable.  Provided it has been properly pasteurised, there are fewer worries about inherent pathogenic bacteria or viruses in pasteurised milk. Pasteurised milk is frequently seen as the norm in terms of food safety and production regulations. Therefore it is easier for the cheesemaker to work within the regulatory framework they find themselves in, rather than taking risks or challenging the status quo.  Lastly, not every aspiring cheesemaker can also become a farmer, or find a good source of raw milk local to them.