According to the article, the limit is not because Softbank and others were not willing to buy more but because Softbank and others could not buy more. Presuma

According to the article, the limit is not because Softbank and others were not willing to buy more but because Softbank and others could not buy more. Presumably because other investors didn't want Softbank to buy too much shares and gain too much control over Uber. In other words, Softbank bought as much as they possibly could.