And why is that? Because they wrestle each other 1000 times a month, that's why! It will become meaningless when they trade wins every other week. It's not because Orton is "nothing", it's because of bad booking. A win over Cena would also become meaningless if he traded wins and losses with a heel on a weekly basis the way Orton's done in the past year.
If Barrett was the only guy who managed to beat Cena while trading wins with him, it would indeed mean something, much like the case with Orton. If Cena lost to every mid-card heel in WWE, only the first couple would matter and the rest would mean little to nothing.
That being said, Orton isn't nothing, but he's at the bottom of the totem pole when it comes to top guys/current stars that mean something in WWE. The list isn't very long of course (Cena, Punk, Ryback, Sheamus, and then Orton is all I'd put right now... and I guess Show since he's WHC), but Randy's star has fallen far from where it was even just a year ago, which of course, as you pointed out, is due to bad booking. Orton's lost to pretty much every heel WWE has right now, and has lost several times since his return in August. He's lost to Ziggler 2-3 times, Barrett twice, Show twice, and has tapped to Del Rio a couple of times in matches. WWE doesn't book Orton like they do the likes of Cena, Sheamus, and Ryback as top faces and Orton isn't The Rock, who can take several losses but due to his overwhelming charisma it still makes the fans care when he loses to someone. Honestly, when Punk and Orton had their match on Raw a couple months ago, I was genuinely shocked they didn't just have Punk go over Orton since every other heel has seemed to.
On the other hand, it's part of the appeal of Orton imo that he's not booked like other faces, and personally I wouldn't have it any other way character wise goes. But even though a lot of people, including myself, complain about how few losses Cena and Sheamus take (we'll pardon Ryback for now since he's relatively new), Orton does it way too much to compensate for their lack of losses. If say, Orton lost to Sandow and Barrett once or twice, Sheamus lost to Ziggler and Del Rio once or twice, and Cena lost to Show once or twice in feuds, it would mean more when Orton loses (while still keeping the meaning when Cena/Sheamus lose) instead of dumping all the losses on Orton.
Punk was actually booked very similarly to Orton when he was a face. He'd lose to Ziggler, Kane, Bryan, Jericho, Tensai, etc. multiple times. Though Punk was cushioned by the fact he was champion and won the matches when they counted.
But yeah, Orton's booking as a top face has been terrible.
Speaking of booking, Sandow's been in a bit of a ditch lately. He was eliminated with relative ease at SVS, lost to Sheamus on Raw last week, and lost to Kofi Kingston on SD last week. And the thing is every time it was a standard taunt/set-up into hitting the finisher without any fight from Sandow to avoid it, which makes him look weak. Especially with Kofi considering he almost never hits trouble in paradise after his "BOOM BOOM" taunt, so that's been my main problem with Sandow's recent losses, and will be my problem if the same thing happens on The Main Event. Otherwise the losses themselves don't bother me. He was bound to get eliminated at SVS one way or the other, everyone except Punk and Big Show so far always losses to Sheamus, losing to Kofi in an IC Title match is fine as it's great Kofi is successfully defending the IC Title (albeit, Sandow should be where Barrett is now and Barrett should be in the main event, but that's a whole other problem I have in itself). And losing to Cena is fine. It's all in the execution though.
And on the topic of Barrett, I like how they've had him on commentary, speaking, which is what he does best, as well as letting him cut a great quick promo on SD last week and the little development on Raw with Barrett handing the title to Kofi (though it would've been better if Barrett assaulted Kofi and ref's had to pull him off him, but WWE seems to not like to use that booking with mid-card feuds).
For Smackdown, haven't watched it in a few weeks, but I'll definitely catch the Sandow IQ Test online. It sounds hilarious.