Why has the average price been reduced after receiving bonus shares ?

The bonus shares are credited to demat a/c of existing shareholders for free. Hence, their buy price is considered as zero. After getting bonus shares, the quantity of the shares increases but the value of the investments  remain same,which leads to reduction of average buy price. The holding average is calculated using the First in First out (FIFO) method. 

FOR EXAMPLE: 

If an Investor A , bought 100 shares of a company at ₹ 60.Then, value of investment = ₹ 6,000 and avg. buy price = 60.Now, if the company declares 2:1 bonus, A will get 200 shares for free and his total holding will increase to 300 shares.Then, value of investment = ₹ 6,000 and avg. buy price = 6,000/300 = ₹ 20.
Thus, average buy price of a stock is reduced after the credit of bonus shares to his account.