Let's run through the list shall we?
So yeah, a mix of contract issues and not being interested in a lot of the tallent is the likely answer for why they don't.
*The reason RVD is a yes despite his age while guys like Hernandez aren't is that he's got more name value in WWE. The other guys would require additional investment in order to get over, while RVD wouldn't and is therefore a safer investment.
- AJ Styles: WWE might have an interest, but due to his priorities WWE doesn't hold much interest for him.
- Abyss: I doubt WWE would want him. They've got plenty of monster heels
- Alex Shelley: Had a try out, WWE had no interest in him.
- Anarquia: I doubt WWE are interested in him
- Antony Nesse: See Anarquia
- Austin Aries: tried and failed to get into WWE last year.
- Bobby Roode: WWE may have interest but due to his TNA contract they can't get at him. When it expires they may make an offer. We'll see though.
- Brian Kendrick: WWE fired him. I doubt they still have interest in hiring him.
- Bully Ray: Eh he's been doing great, but he's also 40 so WWE might not want him for that reason. But in either case his contract doesn't expire for another year
- Chris Sabin: I doubt WWE have interest in him. He's good, but probably not worth what they'd have to pay him to get him out of TNA.
- Christopher Daniels: If WWE wanted him, he'd have been hired when he wasn't with TNA.
- Crimson: He's the stereotypical "WWE type of guy" but I haven't seen enough of his work to say if he's got any skill or charisma to back it up.
- D'Angelo Dinero: They fired him. I have no reason to suspect that they'd want him back.
- Devon: Like Rey they might or might not want him back. Either way it won't be an issue for about a year.
- Doug Williams: If WWE were interested he'd have been hired years ago.
- Eric Young: I strongly doubt WWE have much interest. If they did, he wouldn't have stayed with the "I can work more indie dates" clause in his contract.
- Garrett Bishoff: lol
- Gunner: See Crimson
- Hernandez: WWE might have interest in him, but the fact that he's 39 would be a turn off. Maybe.
- Hulk Hogan: lol
- James Storm: See Bobby Roode
- Jeff Jarrett: Wouldn't leave the company he founded.
- Jeff Hardy: Not right now. If he appears to be clean this time next year they'll probably try to sign him again when his contract comes up.
- Jesse Sorenson: I doubt WWE have any interest in him
- Kazarian: If they wanted him, they'd already have him. Or at least, not fired him back in 2005
- Kid Kash: Fired him, and if they wanted him back, they'd have done it years ago.
- Kurt Angle: They fired him because they thought he'd die on their watch if they didn't. The fact that he's seemingly gone crazy doesn't make me thing they'll be hiring him any time soon
- Magnus: If they don't want Magnus, then whoever does the scouting should be shot.
- Mark Haskins: I doubt it, but I haven't seen enough to say for certain.
- Matt Morgan: They'd probably like him back.
- Mr Anderson: Yeah, they fired the ********. I doubt he's endeared himself to them since.
- Ric Flair: Lol
- The two Ronnies: No. Just no.
- RVD: If the price is right, they probably would take him back. See: Booker T.*
- Samoa Joe: That ship has long since sailed.
- Scott Steiner: Possibly but I doubt either party would be that interested. Also, I cannot imagine him passing a piss test.
- Shannon Moore: I doubt it.
- Zema Ion: Maybe, I haven't seen much of his stuff though, so don't quote me on that. However, I doubt he's going to be on the market for some time.
So yeah, a mix of contract issues and not being interested in a lot of the tallent is the likely answer for why they don't.
*The reason RVD is a yes despite his age while guys like Hernandez aren't is that he's got more name value in WWE. The other guys would require additional investment in order to get over, while RVD wouldn't and is therefore a safer investment.