Why the transmission is done through under ground cable?

Underground transmission due to cables is costlier than overhead transmission as the ground needs to be excavated.  This can be difficult when passing though geographic obstructions such as hills, marshes and rivers.  Special trenches need to be constructed when passing through loose soil.  Besides, heat dissipation in underground cables is an issue. Hence,  the conductors have to be thicker.  The insulation required for the cables is expensive.  but when you enter the city you need to pass the cable in underground method

Its a matter of who can afford the u/g lines.

Reason for undergrounding lines is, safety, it does not disturb the beauty of cities, it needs less maintenance. Its not fussy with tangled wires. In Blizzards, gusty winds, its safe.

transmission is done by under ground cable in population area and having high conductivity

It is safe and clear...................................but its expensive

Hi eng done d under ground transmition done duo to hard work mintenance need it if cable is ded or earted

Reduce heat loss, maintenance less, safety to public,to protect with any ub normal natural effect like cyclone

It depend on the Climate of the Country And Voltage Rating of the Transmission.
In Middle East mostly are under Ground Because Flood is Rarely happens.
In America And Asian Countries is not advisable because of Tropical Weather. 
Over1000vA Transmission cannot be lay under Ground Due to static reacton of on Soil Moisture.
Most of High Voltage Transmission "Like200kvA are Fitted Over Head.