The muslims, after they had expelled the crusaders from the land, purposely desolated and deforested the land, especially the coastal area, so that it would be difficult for another crusade to take another foothold. Over the years the few muslims who lived on the land kept ruining it, deforestation, hunting, over grazing by nomad bedouins. The result was that the land became barren, or covered in deserts, or swamps.
Even today if you can notice that for the most parts the Shomron and Judean hills are bear whereas the parts that are under Israeli control like the hill country around Jerusalem, the Galilee etc. are again covered with forests or gardens.
Here is what British High Commissioner of Sinai, Sir Claude Jarvis, suggested as his explanation in his book "Three deserts" (1936):
"The Arab is sometimes called the Son of the Desert, but, as Palmer said, this is a misnomer as in most cases he is the Father of the Desert, having created it himself, and the arid waste in which he lives and on which practically nothing will grow is the direct result of his appalling indolence, combined with his simian trait of destroying everything he does not understand. A great part of the country in which, he now ekes out his haphazard existence was at one time fairly prosperous and productive and, by failing to repair damage done by wear .... In his campaign of destruction, the Arab has been most loyally supported by his animals, the camel and the goat."
From: Three Deserts (1936) – British High Commissioner of Sinai, Sir Claude Jarvis