They were excellent rifles, but in a market flooded with good bolt rifles, I think Marlin needed to push them for a bit longer than the couple of years they were available. Shooters can be unbelievably cautious about new rifles and cartridges. They'll wait to see if they "catch on" before they buy, and if the company cannot or will not wait out the adjustment/familiarization period, they'll kill the design. Then we can all say "Waaaaaaahhhh! I was gonna buy one of those .356 Marlin 336ER's any day now!"
Oh wait...that's me!