Because he continually gets in front of reporters / writers and tries to continue to spread his ideals and influence even after we found out he's evil. Because he spawned and the spawn is now in politics mirroring his claims and motives... because... he should be gone from the influential political scene... but he isn't. And because, lastly, the jury is still out on how corrupt he was, largely due to their bending of every rule we have in place that is supposed to ensure that people like them don't do the things they did and get away with it. Sadly, he was so good at it, we will probably never know just how bad a man he really is.
They did a poll and only 5% of American admitted to voting for Bush-Cheney. Just kidding, but it may happen;.
Cheney does have extensive knowledge and experience with the situation. He does represent an opposing viewpoint and alternative strategies. If he does not express his ideas they won't even be considered. So speak away you crazy old b*stard.