I'll take a stab at that...
re: "The question was, why are the companies associated with 3-D printing failing to thrive?"
If you look at 3D printing's overall growth as well as sales growth in the individual companies, they are thriving...particularly on the industrial side. Share prices have been hit because the stocks were driven to unrealistic heights during the "hype cycle" of 2013 and still may not be done correcting- especially if we're entering an overall market downturn (which we're overdue for probably).
re: "What is holding back acceptance and sales?" If you're talking consumer grade printers, I think it's the learning curve, need for new, innovative materials, and the still relatively slow speed of 3D printing. It's still a niche market. To cross into mainstream adoption for consumers we need much faster print speeds, true "plug and play" functionality, and better materials (much more than ABS and PLA plastics) to print with.
Just my opinion.
