Why do we use star connection on transmission and distribution on delta?

The star connection is mainly required for the Power Transmission Network

for longer distances, whereas in delta connection mainly in Distribution networks and is used for shorter distances.

For long-distance transmission and distribution, star connection is preferred because it requires little insulation and has a neutral that helps to balance the circuit.

Delta connections require more insulation and have issues with unbalanced currents, hence they are preferable for transmission and distribution over shorter distances.