"Supporters of the District’s voting-rights legislation predicted that a similar amendment could be kept off the House bill. As long as it were in only one version, Democratic leaders could strip it out in conference. Conference reports can’t be amended, just voted up or down, so the conference report would pass without the gun bill.
The Democrats removed the bill from the floor because they did not want vulnerable members to have the NRA giving them bad rankings which would hurt them in 2010. The article comments that the "Democrats may be running the House, but the National Rifle Association (NRA) can still stop a bill in its tracks." Clever use of House rules.