Here's why:
* Semicolons are used to connect two independent clauses that are closely related in meaning. "And" usually indicates a less complex connection between clauses, making a comma more appropriate.
* Using a semicolon before "and" can be grammatically incorrect unless you are using it to separate a series of items, each of which includes a comma.
Examples:
* Incorrect: I went to the store; and I bought some milk.
* Correct: I went to the store, and I bought some milk.
Exception:
* Series of items: The guests included the Mayor of the city, a famous author, and a well-known philanthropist; and they all enjoyed the event.
In this case, the semicolon is used to separate the entire series of guests from the following clause.