From baseball-reference.com -
http://sabr.org/bioproj/person/3b7d0b88
He missed much of the 1918 season with illness but regained his starting position the next year, appearing in 100 games at second base and batting .289 with seven home runs. The 33-year-old Doyle remained a regular in 1920, closing out his major-league career by batting .285 in 137 games.
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It also says in this brief bio he and Mathewson watched the stock market closely, and Doyle bought real estate in Florida, so perhaps he was financially set and no longer needed the baseball money. He also turned down a 2 year, $27K contact to jump to the Federal League, to stay with the Giants for $8000, so perhaps he retired early due to illness, and being financially well off.