Due to following reasons, AC systems are preferred over DC system:
a) It is easy to maintain and change the voltage level of AC electricity for transmission and distribution.
b) Plant cost for AC transmission (circuit breakers, isolators, transformers etc) is much lower than the equivalent DC transmission.
c) If we use dc transmission, rectification and inversion is required at source and sink respectively which is a costly affair.
d) From power stations, AC is produced so it is better to use AC then DC.
e) The important factor is that AC can be easily converted into DC by using a Rectifier . but Conversion of DC is very difficult
f) When a large fault occurs in a network, it is easier to interrupt in an AC system.
A comments collected from internet about this matter: With increase in voltage, line current is reduced for transmitting same power to long distance. Hence changing the voltage level is required for loss less transmission. AC transmission system has the property of transforming the level of voltage and current without change in frequency and power with the help of transformer.