You should check out this thread asking a similar question; https://www.reddit.com/r/AskHistorians/comments/4p4qt7/why_is_the_fertile_crescent_now_desert/

Especially the answer by /u/brigantus

Fertility is a relative concept. When we talk about the fertile crescent, we mostly mean that it's fertile in relation to the surrounding deserts of Southwest Asia. Fertility is also relative to what you want to get out of land and what technology you have to do so. From the point of view of Europeans with modern agricultural technology, Mesopotamia isn't very fertile, no. But in the context of prehistoric farming it had several distinct advantages:

• The vast majority of the crops and animals used by prehistoric farmers were native to Southwest Asia, and therefore naturally thrived in its climate. By contrast, contemporary farmers in Europe were struggling to get plants that evolved in a semi-arid, Mediterranean climate to adapt to a soggy temperate one.
• The landscape of Mesopotamia is very open and the soils are soft and friable. You don't have to break your back clearing woodland for fields (again, contrast Europe) and you don't need a plough to break up heavy clay soils.
• And the big one: floodplains. Whereas farmers in other regions would soon run into the challenge of maintaining soil fertility in the long term, the Tigris and Euphrates delivered fresh, fertile silt to Mesopotamian fields on a yearly basis. It's no coincidence that the earliest urban societies appeared in the floodplains of Mesopotamia, the Nile and the Yellow River.