Because there's a profitability curve for installing winglets. Mind you, these are pretty heavy, tall things. They adjust the aerodynamic profile of the plane slightly, thereby increasing efficiency of fuel-burn. This efficiency occurs only at flights above a certain break-even distance. Given the purposes for airlines like AA
to retrofit their 737NGs with winglets, I'd say this breakeven point is somewhere around 1200-1500nm.
737-600 are Boeing's unfortuante attempt at creating an RJ
with 737 commonality, never mind that the 737 platform is unfit to compete with modern day RJs (the 737-100 was of course nothing other than an RJ
, but that was in the 60s). Given that RJs don't usually fly distances of 1200-1500nm, it is not profiatble to install 737NG-custom winglets on them. They will cost more than they'll save on fuel burn.
"Get your facts first. Then you may distort them as much as you please" -- Mark Twain