Why was Theodore Roosevelt elected to be McKinley's running mate?

Before becoming a candidate for vice president in 1900, Theodore "Teddy" Roosevelt (1858–1919) served as governor of New York. There are several reasons why he was nominated for the vice presidency.
First, Roosevelt was a very well-known national figure. His military exploits as leader of the Rough Riders in Cuba in the Spanish-American War (1898) burnished his reputation. His charisma and image enhanced the Republican ticket in 1900.
Second, Roosevelt's progressive policies as governor of New York alienated many leaders in the Republican party. They sought to put him in the inactive role of the vice presidency as a way to stifle his reformist tendencies. Mark Hanna, the wealthy and powerful industrialist from Ohio, opposed Roosevelt's nomination, but he could not prevent it.
Third, Roosevelt welcomed the opportunity to campaign against William Jennings Bryan, the Democratic candidate. Roosevelt thought Bryan's policies were too radical.
In 1901, after the assassination of William McKinley, Roosevelt became the youngest man ever to assume the presidency.