Question

why has the government of the India enacted the law to fix the minimum age of marriage as 18 in girls and 21 in boys

Answer 1

the government of the India enacted the law to fix the minimum age of marriage as 18 in girls and 21 in boys in order to prevent child marriage.

Answer 2

The minimum age of marriage for males is pegged at 21 as an indirect family planning method. 

India was facing an overpopulation problem  and the government needed some serious solutions. One of the first blame for the population problem went to the practice of early marriage. Marriage at one's teens opened the most fertile years of a man's and woman's reproductive life to reproduction and QED, a lot of babies.

So by raising the marriage age for men and women, the law made sure that men were past the peak of their libidinous teenage before they married, so that by the time they were in their 20s, the sexual frequency would be reduced enough for men to make less babies (since men were presumed to be THE initiators of sex in the patriarchial cultures). 

It was all Gandhian philosophy, tabled by Nehru while drafting the Hindu Code Bill. Since the elite ruling class and the richie rich had the moolah to get laid before marriage - sex starvation was the burden the middle class Hindu men to bear.

As a 'side effect', it worked very well for middle class parents. What with their sons suffering involuntary celibacy for decades, they were eternally grateful when their mommy got them any 18 year old. They were too grateful to be picky, on any level. The Great Indian Culture (TM) at work!

And second season just for after this boring speech...

Possibly because you can't ask an 18 year old Male "How much does he earn?" You'd need 3 more years for graduation at least,
But you can ask an 18 year girl "Can she cook?"