Because the west are the people that are asking Brazil not to take advantage of one of its natural resources. Everyone else has had the luxury of exploiting their land and we'll suffer as a result if Brazil does as well.
P.S. My country has been planting vast areas of forest since the 13th century for economic purposes. Not sure what you mean by "Everyone else has had the luxury of exploiting their land", since here (and in most of the EU) we have vast areas of forest and actually took care of them.
I can even accept that richer countries could pay somewhat more. But, still, there are a lot of non-western countries that are quite rich. So, why only western countries should pay?