Comparing us unfavorably in this regard to Milwaukee is stupid. I don't know why they had 40K for one game, but their average attendances is below ours this year.
We are a small market. 20K for April isn't that bad.
Also, don't expect the attendance to get much better in 2006. It continued to be bad in 1999, even though by the end of the year the Reds were one of the most exciting teams in baseball. The general consensus then was that a spike in attendance caused by winning would be felt more the next year. (And attendance did go way up the next year - but obviously that was Griffey's first year, so it's hard to tell what effect 1999 had.)