I don't think they weren't "invited" as much as they chose not to show.
Bush: GWB knows that he taints Romney's chance for reelection - as the Dems continue to use the "Blame Bush" line cause it's all they got (getting old after 4 years).
Cheney: He knows he'd sour the chances of Romney.
Powell: He voted for Obama.
But they did trot out Condi Rice.
The others probably thought it was best they stayed out of it.
Ok, so let's turn the tables and ask the same question: Where was Nancy Pelosi, Harry Reid, Al Sharpton, and Jesse Jackson? How about Al Gore? Did any of THEM have a pivotal role in the DNC Convention? Nope. Yet, they trot out Slick Willie - a great example of a guy with great principles and character, a cigar officiando and a family man who honors his marriage...How quickly we forget and are willing to overlook...He has about as much chutzpah as Obama does. It would be pitch dark and they'd say the sun was out and some people would be foolish enough to believe them...so just keep drinking the coolaide...
Both parties kept those out of the Convention that they felt might not help their chances. Neither are innocent of that so your OP really has little meaning.
Bill Maher makes his money by spouting some serious liberal BS. He does not allow any alternate voices on his show. I'd like to see him let Chris Christy on there (who would eat him a new one and maybe bioch slap him off the stage). He has said stuff about his political rivals the FAR outweighs the level of decency (even more than Rush - and I think Rush goes too far too sometimes). He has zero credibility and he couldn't find his azz with both hands unless he had someone to hand it to him. Given he's broadcasting from California explains a lot of it, as CA is certainly the most enlightened, informed, prosperous, and financially stable state in the country (NOT).
What I am worried about is that the current climate and level of vitrol is SO SEVERE that Congress can not and will not act and would rather let the the gates of hell open, than either side find compromise until it's too late...