That isn't why at all
The Yankees have pretty consistently been a 90+ game winner and playoff team so the additions they made in the off-season were expected to make the team even better than it was in that prior year
When the Jays or Marlins make some big off-season acquisitions, there is doubt and rightfully so of what those additions could do to the team because of the year(s) prior when the team did not finish well (perfect example in the NHL would be the Wild with this past off-season)
The Rays and most recently the O's have been names to be reckoned with coming out of the AL East along with the Yanks - the Sox haven't been in the playoffs in 3 years, they haven't been a threat at all recently - you're talking about the best division in the league, where this coming season each team is capable of winning 80+ games
Baseball has to be the sport were team play isn't all that important. If you have solid talent at the plate, on the mound and can avoid the injury bug, you have a solid shot at making the playoffs if you're dressing a superior lineup then the team you are facing.
The Jays are making all the right moves ATM and they are contenders when looking at their lineup... on paper. Now, they just have to execute on the field and the results should be there.
The Yankees have been on top of the world for years cause they've been investing more money then any other baseball team in the league, period. When you dish out cash like they do, it just has to pay off and the results are there to prove it. It's no surprise that the Royals have been bottom feaders since god knows when.
The only exception is the Oakland A's. They have a tight budget and always compete for the AL West but like in anything, there's always exceptions to the rule.