Perhaps because the deck is stacked against the owner: "The government must initially prove by a preponderance of the evidence [NOT beyond a reasonable doubt] that the property is subject to forfeiture. The owners must then prove by a preponderance of the evidence that they had no knowledge of the underlying crime." - http://www.nhbar.org/publications/display-news-issue.asp?id=7171
Its just too expensive and time-consuming to contest a civil forfeiture.
After all, you have to prove you aren’t guilty. Just the way corrupt cops and prosecutors like it.