As for the Yankees, they have the advantage of playing in a largest market in North America, has its own TV station and is the most well known MLB team in the world so their bankroll is more than many teams combined)
The Boss and company deserve credit for the 90s Yankees since they drafted well (Jeter, Posada, Pettite, Rivera, Wiliams, Soriano) then added a lot of quality talent (Tito, Boggs, Girardi, Rogers, Wettland, Key, O'Neill)
When they started spending bigger money and creating huge contracts (Arod, Burnett, Abreu,Giambi) is when things started to decline IMO. However, credit to the BOSS as he wanted to win badley and spending wasnt an issue.
Cheers, K