Daver has it exactly right.
Marketing certainly helps but when you cut through everything the bottom line is that nothing is a better marketing tool than winning...and I don't mean being in first place for three months after a losing season the year before...to add to the fact that this franchise hasn't even had back to back winning seasons since 2005-2006 (I wanted to preempt the "it's all the Sox fans fault crowd...")
Again I go back to EE statement soon after he and JR bought the franchise (paraphrasing since I don't have the exact quote in front of me...)
'The way to win today is with free agency and trades...'
They have never believed in the minor league system / approach to building a consistent winner, they have never invested the resources like other franchises (Sox spent the least amount of money in minor league bonuses in the last five years of MLB) and just in the last decade they can't even hang on to the same minor league director for longer than three or four years. Hard to develop a consistent development approach when the folks in charge are leaving or getting fired every few years.
Then you add in the instances where JR intervened in who the Sox were going to draft and his dislike for certain agents (who usually happen to control very good talent) and you are shooting yourself in the foot.
If the Sox in fact invested heavily in free agents (the TOP guys) and made good trades I'd say the lack of a minor league system wouldn't be a big deal. But not only do they chisel the minor league system but they are falling flat in the other two areas as well.
Not a good situation.