I will try to provide more information besides the obvious reason already provided in the comments:
keep and preferably increase his power
Of course, one should not expect from an official to publicly recognize this as the main reason, but some of his declarations might be useful. According to Telegraph:
- showing working examples:
Mr Erdogan responded to arguments that putting political power in the
hands of the presidency would not work in a "unitary state".
He said there were other examples of its being successful. "There are
already examples in the world,” he said.
“You can see it when you look at Hitler's Germany. There are later
examples in various other countries."
"What is important is that a presidential system should not disturb
the people in its implementation,” he said. “If you provide justice,
there will be no problem because what people want and expect is
- it is not that wrong because it also happened in parliamentary systems (Whataboutism):
There are authoritarian structures coming out of parliamentary
systems,” he said. “Hitler’s Germany was born out of a parliamentary
This last point is a bit ironic since President Ergodan managed to increase his powers through the popular vote in 2017 (referendum).