Allen was a back the Bears turned to when Ennis busted out. He was just a substitute from whom little was expected but turned out to be a pleasant surprise. Thomas was rookie of the year, led the Bears in rushing his first three years and averaged about 900 yards a year. This was not good enough and they let him go.
Then they must have been charging too much, which is fine, but that isn't the case with Bus. He deliberately made himself affordable and does plenty to earn his salary, including stepping up to the plate to become the feature back last year when Staley got hurt and finish the job of getting their run oriented offense into the playoffs. And given how weak the Bears offense has been for the last few years they've done a fine job of establishing that they don't actually have a good eye for talent, so frankly "not able to make the Bears" is a compliment.