I think the big GC guys are all basically the same - they're all doping, they all know the others are doping, and they'll all deny it if asked. The difference with Froome is:
a) he took longer to get on the sauce so he has an incriminating jump in performance to explain away, but mostly:
b) he's more or less dominated the Tour for pretty much the last four years, which gets way more attention than any other race
c) this period coincided with Tyler's book, Lance's ban and confession, all the rest of it
All this means Froome gets a lot more scrutiny than the others, and the more you're questioned the more you have to deny.
I think a lot of people see Contador and Nibali, who are stylish riders from traditional cycling countries, and buy into the talent angle - the idea that they'd be natural champions if there was no dope in cycling beyond a nice cappuccino. They see Froome climbing with all the grace of a daddy long legs (the crane fly, my American friends, not the spider) and think "Him?".
The problem Froome represents for these people is that he strips away whatever romance is associated with the sport and leaves only one fact: watts win (grand tour) races, and style and panache, all that stuff, never meant anything anyway. The difference between the top riders is so small, and the difference high-octane doping makes is so big, that the only way we'd ever really know would be to lock them in a room for a year with a bike on rollers each, and push food under the door, and send them off to race.
And then we'd know.