Ellsbury and Buchholz are both young, talented, and cost-controlled. Buchholz is having a rough season, but he has a track record of being a tremendous young prospect. Ellsbury is having a good year (I'm not sure what the expectations were) and is definitely the type of run-creator our offense can use.
It's the "cost-controlled" part that's most important. With the Sox having a top-heavy payroll, they need to have some contributors that don't make an insane salary. Between the two of them, Ellsbury and Buchholz are making around $800,000 this season. That's a bargain. Santana, on the other hand, is making $17 million. The Mets had to pay him $137.5 million over six years, for a guy who is already 29 years old. A contract like that for a pitcher is a huge, huge risk; if his arm blows out, your payroll is wrecked for half a decade.
I'm quite happy that the Sox didn't make the trade. The most reported deal was Lester + Ellsbury + another prospect (either Bowden or Masterson, usually) for Santana. I'd gladly bank on their future, plus have an additional $20 million per season to spend, than I would add Santana.