Why is entrepreneurship the most important factor in economic development?

There is no evidence that that is true.

The evidence is that the most important single factor in economic development is peace. If a war is fought in your country that does more damage to your economy than anything else can overcome.

The second most important factor in development is honesty of government. A kleptocratic, confiscating, bribe-extorting government is worse than anything except war.

The third most important factor is found to be having a court system that quickly, reliably, and cheaply enforces contracts without favoritism even to the wealthy.

The fourth, almost equal to the third, is adequate transport infrastructure that lets commodities be delivered quickly, reliably, and cheaply.

And then there is education, in particular literacy.