Imagination is the mental power to imagine – the ability to create images in one’s mind. Creating mental pictures is not necessarily artistic creativity, and this is what might be tripping you up. Simply creating a picture in your mind is an act of imagination, even when the picture is as mundane as a drainpipe or a traffic sign.
Understood in this light, Russell is only saying that governors often are so far removed from what they are governing that they have no practical mental picture of it.
He goes on to argue that bureaucrats really only have a practical mental picture of the rigid environment in which they work. This is why, in Russell’s view, bureaucrats tend to impose rigid schemes on whoever they are governing, regardless of whether that interferes with creativity and self-determination and leads to a stifled culture. It is the only way of life that they can practically imagine:
This makes them ignorant of much that they ought to know, even when they are industrious and willing to learn whatever can be taught by statistics and blue-books. The one thing they understand intimately is the office routine and the administrative rules. The result is an undue anxiety to secure a uniform system. … The result inevitably has something of the deadly dullness of a new rectangular town, as compared with the beauty and richness of an ancient city which has lived and grown with the separate lives and individualities of many generations.
—Bertrand Russell, Political Ideals