The reasons are as follows:
The MySQL extension:
- Does not come under active development
 
- Is officially deprecated as of PHP 5.5 (released June 2013).
 
- Has been removed entirely as of PHP 7.0 (released December 2015)
 
- It lacks an OO interface
 
- Also does not  support:
- Non-blocking, asynchronous queries
 
- Prepared statements or parameterized queries
 
- Stored procedures
 
- Multiple Statements
 
- Transactions
 
- The "new" password authentication method (on by default in MySQL 5.6; required in 5.7)
 
- All of the functionality in MySQL 5.1
 
 
Since it is deprecated,  I would suggest not using it, as it would make your code less future proof.