'because of the difficulty' is correct and can be used in written English.
For example, "I decided to ask for help because of the difficulty of the task."
That is partly because of the difficulty of growing asparagus.
They generally run parallel to the rivers because of the difficulty of constructing bridges.
Because of the difficulty of measuring the brain, psychiatric diagnoses are still mainly based on symptoms.
The caves remained unexplored for so long because of the difficulty of probing their flooded interiors.
Partly because of the difficulty of ensuring additionality, many offset providers guarantee their emissions savings.
Customarily, none have been assigned because of the difficulty of getting troops to the sites.
"It was special this week because of the difficulty of the course.
The overall power project has been repeatedly delayed because of the difficulty of security and logistics.
Also, I'm not using names because of the difficulty of verifying e-mail signatures.
The pool of potential buyers is also drying up because of the difficulty in obtaining credit.
The whole area of derivative trading is fraught with uncertainty because of the difficulty of valuation".
