Why was it still difficult to travel from al-ain across the desert

Because of its terrains and weather.

Explanation:

Due to the sand composition in the desert, it is very easy for cart wheels to be stacked and make the traveler trapped in the middle of nowhere.

On top of that, the weather on the desert were extremely hot during the day and extremely cold during the night. In order to survive this, travelers need to made a through preparations (such a carrying large amount of water, preparing for oil/fire to survive the cold weather, etc)  that made the travel became quite expensive.

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