Well first off money. The Yankees always outspend everyone in the major leagues (the Mariners are 18th in payroll).
But also: Last week the Mariners cut Aaron Laffey, a 26 year old lefty pitcher who has both started and relieved in the major leagues. His crime: having a 4.01 ERA in 36 relief appearances with a 1-1 record. His WAR was .3 as a Mariner (their all-star closer League has a Wins Above Replacement of .9, Mariano Rivera has a 2.8). He was helping them more than he was hurting them, is young, and could be a starter for them in the future. Not to mention, they cut him right before rosters expanded. The Yankees of course, gobbled him up and are more than happy to have the lefty in the bullpen. This doesn't have anything to do with money either. Laffey's contract:
1 years / $431.60k