I would think that the main reason that Christopher Daniels is not currently in the WWE is actually pretty simple: he's simply not that good.
As a general rule of thumb, when guys are past their prime, out of shape, no longer relevant, or whatever other situation arises to cause their departure from a company to end up in the other one, it's usually in the WWE to TNA direction. We tend to see a lot of "WWE rejects" end up in TNA, whereas we don't tend to see a lot of "TNA rejects" in WWE.
Which actually makes a lot of sense. Let's face it, if your deemed not up to par by TNA standards, you're hardly WWE-calibre material. Because by looking at TNA over recent weeks, they seem to have a reasonably high tolerance for irrelevance and incompetence on their roster. If you cannot make it here, chances are your career is essentially over.