Well, the population of the towns of Gerlock and Empire would be financially devastated if they did that. See, up road 447 is basically a few ranches and some old mining towns full of older folks who have lived there for decades and are now largely stuck there. Empire was largely supported by a gypsum mine that closed 3 years ago. If BM didn't occur once a year and inject about $5-10m into their economy, those towns would entirely collapse. The BORG is sensitive to that but over the last few years local government officials have become more and more greedy (the brand new $75,000 police cars for a town of 500 people didn't help). If the BM decides to pull up stakes and move, that area will be ruined and the locals will suffer greatly.
Also, the Black Rock Desert isn't really something that needs protecting. The leave-no-trace ethic is more of a political statement than a practical effort at 0 trace. Many people have misinterpreted this into an absurd standard. The rest of the year on the Black Rock Desert is largely unused except for the occasional small rocket launch, attempt at a land speed record, or a filming commercial that usually leaves about as much trash behind afterwards as the week of BM does. Largely due to the fact that they don't have to care if they litter as the local government is dependent upon them to get a bit of $$ to help support a largely failing local economy. Its absurd to think 50,000 people can be in one place for a week without leaving any mark. Its also absurd to think that BM isn't the closest thing that we have in the US to a true leave-no-trace event. If you've ever been around after a large event and seen entire stadiums covered in trash after a 2 hour concert, then the bit of trace BM leaves after a week is absolutely amazing. If you want to blast them for that, they you are entirely missing the point (perhaps they are too), but the ones that put the idea of leave no trace in to their head have accomplished their goals and then some. Its the cleanest gathering of people in the US every year when measured by the amount of trash or trace left afterwards. That's actually pretty amazing.