In a movie year they order about 25 cases too many of the same exact wave in a small region that can not support the numbers long term. So, they end up with a surplus of pegwarmers that will never move off of their pegs. It's not really do to the fact that the figures did not sell it's just that they do not know how to order worth a crap. Then it's the "Ohhhh, that Star Wars stuff just does not sell. Get rid of it."
Well, and Jebus forgive me for defending Wal-Mart here, the problem in the case you're referring to DB comes not totally from WM, but also from Hasbro's case assortments. From this point on, I'm going to be referring to a single WM store instead of the entire chain, just for the sake of clarity.
Hypothetically, WM decides that it is going to devote 24 pegs, each of which holds about 6 figures, to Star Wars figures. That is 144 figures, or going by Hasbro's ratios, 12 cases (12 figures per case). The problem is that Hasbro does not enable a salesperson for the major chains to say, "I want to order more of Case Assortment 97800.B"; their only option is to say, "I want to order more of Case Assortment 97800." They could see pegs devoid of Collection 1 figures except for Neimoidians and Clone Pilots, but reordered cases contain more Neimoidians and Clone Pilots. So, while the figures that people want (Clones, Vader, etc) from those cases disappear, more and more Neimoidians and Clone Pilots are shipped, and they add to the glut already on the pegs.
When inventory is done, the employees at individual stores couldn't care less WHO is on the pegs as long as SOMEONE is on the pegs. All they see are "We have 120 units of UPC 97800" and not "We have 80 Neimoidians, 40 Clone Pilots, and nothing of anyone else." And the problem that comes up there is that when it comes time to reorder, they see full pegs of figures and don't order more. The pegs are full with the same two figures, but it's all Star Wars to them.
The surplus is because Hasbro still cannot figure out how to do market research outside of their mythical focus groups they spout off about every time someone questions them about their silly ratios. Retail is choking on Clone Pilots and Neimoidians? SHIP MORE. Grievous' Bodyguard and Neimoidians dying at retail? REPAINT THEM AND SHIP MORE. Clones nonexistent? "A solid case of Clones would be retail suicide!" AND MORE NEIMOIDIANS ISN'T?

So, you've got Hasbro shipping the figures that are clogging the Wal-Mart pegs over and over due to Pawtucket existing on an alternate plane of reality, and WM who are ordering things based on a uniform UPC that they have no say in which case assortment they can get, and both of them are contributing to a glut of the same six figures that are preventing more figures that would sell from being ordered.
The Wal-Mart that is closest to my current place of residence has 20 pegs devoted to SW figures right now. As of earlier today, the only figures they have on these 20 pegs, filled completely full with no room for new stock, are the following:
-Wookiee Warrior (43)
-Neimoidian Warrior (42)
-Clone Pilot (34; Gray)
-Supreme Chancellor Palpatine (14)
-Palpatine (35)
-Grievous' Bodyguard
-Bail Organa
NOTHING ELSE. And we're talking a mountain of the first three figures in particular that I don't think clearance could even make a dent in at this point. But from WM's perspective, the pegs are full and that's all that matters to them. As the months go by though and those same figures are there, it won't be entering their minds that these figures aren't selling, it's that "STAR WARS AIN'T SELLIN'." This gets reported back to Bentonville, and I'm sure a large portion of other WMs across the country make the same report, and WM starts to wonder if they need to be in the SW business anymore.
Is there a simple solution? No, there's not. Could things be done to lessen the pegwarmer problem? Well, Hasbro could always keep up on what is actually selling vs. what isn't at retailers, but that's just crazy talk, isn't it? After all, it's all retail's fault that we can't find what we're looking for.
People have pretty much been unanimous on the pegwarmer problem being better this time around than with Episodes I and II, and for the most part I agree. The problem that I'm seeing now is that while the other two collections had their major pegwarmers, there was still a variety of characters to choose from and main characters and army builders were available. This time around, it's the same 8-10 characters of dubious importance, and main characters are nonexistent. So, while the pegwarmer problem isn't as bad as the first two, it's worse in a totally different way.
Again, I hate to say it, but the day that Wal-Mart puts its foot down and says, "Enough is enough" is the day that Hasbro literally does what they've been doing figuratively as of late and kills the line completely.