You can only push so many people at one time. If you're pushing everyone to the top of the card all the time, who exactly is the undercard?
The top of the card is bloated already.
Locks: Cena, Punk, any big name part timers (taker, hhh, lesnar, rock, etc)
One step down: Orton, Sheamus, ADR
On the way: Bryan, Ziggler, Ryback
Getting there: Rhodes, Sandow
Vets: Kane, Show, RVD, Jericho, Henry
Fallen stars: Miz (beat Cena, as a heel, in the main event at Mania, that alone trumps most accomplishments).
Where does Barrett fit in? Is he as talented as Bryan or Ziggler? Absolutely not. Does he have the freak size/strength of Ryback? Does he have the multiple world title runs to point to like Orton, ADR, or Sheamus?
Barrett is bland. He has no real character outside of being a heel foreigner. His promos don't have direction, he seems to have no intensity in his delivery. His finisher is weak. His matches are above average at best (and with the recent string of good matches lately from other guys looks average or worse in comparison). Without the Nexus, he's not really been interesting. He tried to play a tough street fighter from the UK, does the mean streets of london sound anywhere near as intimidating as brooklyn? I think Dan Henderson showed just how tough fighters from the UK are when he made Bisping his bitch.
As far as what to do with him... Who knows. Pairing him up with Coulter might give him a chance to succeed. Especially if they had a couple more foreign heels to add in. Really though, I'm not concerned with him.
Bryan, Cesaro, and several guys in NXT are much better than him in the ring, a lot of guys are better on the mic. Barrett had his chance to cement himself as a upper tier guy and for one reason or another didn't get the job done.
In the attitude era, you had Austin, Taker, Rock, Foley, and HHH feuding over the title, throw in Jericho, Show, and Angle in the latter part. You had Eddie and Benoit when they jumped ship as just below world title contenders and the rest of the roster were below them. Sure you had Kane for a little while as well, but that didn't last.
Top guys are top guys for a reason, generally there is only so much room at the top. You can't have an entire roster filled with main eventers as not every position on the card is a main event. Why is Barrett above the US or IC title? Why is he above being an enhancement talent? Is he really that special?