Because of price increase and availability issues. The camping game has become more complicated in the end you must set your own criteria and priority.
Is price your main priority? Are you willing to camp most anywhere if the price is right? Public parks are much cheaper but because of increased demand, reservations must be made months to years in advance.
If you must be in a specific area at a certain time you need to be prepared to increase you budget.
It's all supply and demand for the most part, if you are not willing to pay the increased price the next guy in line will be willing to pay.
If your schedule is somewhat flexible you can save by choosing alternative locations and dates. However if you schedule is fixed be ready pay the market rate