I think part of it is only four fan bases are left in the NBA (Boston, Orlando, LA, and Phoenix). Now LA and Boston's fan bases are huge. Orlando's is relatively young and had its most success ever last year. I have no idea what type of fans Phoenix has.
But with LeBron you affect the entire league. Especially Chicago, NY, LA (Clippers), Miami which are five of the top 10 media markets. Let's also not forget that Atlanta could offer him a deal which is another top 10 market.
Boston and LA are top 10 markets, Phoenix is not, I don't think Orlando even cracks the top 20. But, between Chicago, NY, part of LA, Miami, Atlanta, and Cleveland, that's a lot more fans interested in LeBron than the NBA Playoffs.