The cars handle fundamentally differently, and it has been shown on quite a few occasions that some just can't cross from Champ Cars to F1. To compare Champ Cars/IRL and F1 and believe you can pluck drivers from one formula to the other is a mistake that many make (especially the teams for some reason), they might as well be doing it with F1 and Rallying.
That is absurd. Just ask Nigel Mansell, Justin Wilson, Robert Doornbos, or any GP2 or F3000 driver who has ever graduated to F1 for that matter. While Indycars and Champ Cars are both heavier and less powerful than F1 cars, the priniciples of preparing and driving a high-powered formula car are essentially the same. The weight/HP ratio of an Indycar is comparable to F3000 or GP2 cars, and Champ Cars (where Bordais got his experience) have a lower weight/HP ratio than GP2 cars. And back when Michael Andretti got the huge McLaren shaft, the Champ Cars were actually making more power than F1 cars...
He has been given a fair chance and hasn't cut it - time to go I'm afraid.
He's doing basically as well as his other teammate, which was also the case with Scott Speed if you remember. The bottom line is that if you don't outperform your teammate on a regular basis that you can always be the victim of being cut, regardless of how good you might actually be. In this particular case, I think they are really blaming the driver instead of the car and the preparation where most of the blame belongs. Once again, quite similar to the way Speed was handled by the same team under basically the same conditions.
And let's look at Villeneuve's record after Williams fell apart. If he hadn't won an F1 championship, everybody would be saying the same about him.
But such is the way with F1. It's all about "what have you done for me lately", and not how good of a driver you might actually be. Until this year, both Jenson Button's and Mark Webber's abilities were always under scruntiy as well, and some people are even now claiming that Barrichello can't drive very well despite his long and distinguised career.