Why did William McKinley win the election?

A confluence of events paves the path to the presidency. Personality, resume, political connections, and a little luck of being the right person at the right time characterized McKinley's rise to the highest office in the land!
In the first election, the country was mired in an economic depression. The democratic candidate was a capable and a formidable opponent, William Jennings Bryan. Bryan was known for his oratory ability, populist views, cerebral arguments, and support of progressive ideals. Bryan had been nominated to run previously and was a strong campaigner. In the election of 1896 against McKinley in the face of economic depression, Bryan supported the advanced idea of what would now be called "easy money." Bryan believed freely minting unlimited silver and gold coinage, flooding the market, would ease the tight money supply. You might imagine explaining how this would benefit the average citizen, weary of economic problems and more concerned with the necessities of life during the depression, might be a hard sell. Though Bryan drew large crowds and was enthusiastically received at every stop in the campaign, McKinley was supported by wealthy Republican supporters and a well-defined political machine. His message addressed the economic issues in terms most people could easily understand.
McKinley was known for being personable and had a reputation as a capable public servant. He served in Congress for fourteen years, earning a reputation as a bright, well-informed politician. McKinley became the chair of the House Ways and Means Committee. He was considered an expert on trade and tariff policy. He left the house to become Governor of Ohio and served for two terms before being nominated by the Republicans as their presidential candidate. McKinley's reputation and experience, along with Bryan's political inability to craft a message that connected on a personal level with voters, resulted in McKinley easily defeating Bryan. The depression had taken its toll on the citizens and the political fortune of the Democratic candidate.
Before the end of McKinley's first term, the American economy had left the depression and was in the throes of a minor economic boom. The currency question was no longer an issue, and McKinley concentrated his efforts on trade and tariffs, raising tariffs to unprecedented levels at the time. McKinley flexed American military and naval muscle when he successfully defeated the renowned Spanish naval fleet at Cuba. The war lasted 100 days and ended with the destruction of the Spanish navy and the expansion of the United States' territory to include Cuba, Puerto Rico, Guam, and the Philippines.
In the election of 1900, the Democrats once again turned to Bryan, who attacked McKinley as being an imperialist and promoting war. McKinley's campaign emphasized the economy. They used the slogan "the full dinner pail," emphasizing the prosperity and future of a strong United States. McKinley easily won re-election. Economic issues in both elections proved to be more favorable to McKinley than to the message of his rival, Bryan.
https://millercenter.org/president/mckinley

https://www.history.com/topics/us-presidents/william-mckinley

https://www.whitehouse.gov/about-the-white-house/presidents/william-mckinley/