Monster is one of my favorite anime, but its DVD release was doomed to fail. If I remember right, the R1 release was a Viz license. Viz at the time already had a reputation for abandoning shows partway (e.g., Full Moon wo Sagashite) and even optimists didn't like the odds of a full Monster release. With replacement music for the ED and rumors of interlaced video, fans of the series were trapped by a Catch-22: They had to buy enough of the early discs to ensure a successful full 74-episode release, but the likelihood of that happening was so low that these fans would almost certainly suffer the same fate as FMoS fans. As you may have guessed, the DVD release as a whole did not go especially well.
Thankfully, the manga release appears to be in much better shape. Although also a Viz release, all 18 volumes did get releases. The last of these volumes came out in 2008, and I'm pretty sure they're out of print now, but Viz currently publishes the Monster manga in large double-length compendiums with the final (ninth) volume due out in July 2016. I haven't actually started reading these yet, thanks to my ever-prodigious backlog, but picking these up is a no-brainer, even if the manga lacks the anime's MAMIKORE voice acting.