I don't think you should take the longevity of each as a mark of popularity or quality. The British version wasn't cancelled - Gervais and Marchant ended it deliberately with a proper conclusion. I don't think anyone doubts more British series would have been commissioned if they'd wanted to carry on.

When Gervais was asked if he likes the American version his response was "Yes of course. It's made me very rich." That's his dry sense of humor, but I suspect it also hints at why they stopped doing the UK version and started on the US version instead.

And it has been said multiple times by people working on the American version that besides the inital setup, Gervais and Merchant had very little if any input and everything was done by the American wtiters/show runners.