basically the show used a huge number of actual songs during the show, everything from phil collins to sum-41. they only licensed the songs to show during the series and its subsequent syndication. they did not license the songs for dvd release, hence if they wish to release the series with the songs intact then they have to pay for them again, or edit the show to not include the music, and this got to be too expensive so they cancelled the release of season 2 back in 2003.
its actually a semi-common problem with tv shows on dvd and its the same reason the wonder years has never been released.