Let's break some marketing concepts down analytically, really quickly from a high-level:
From a product standpoint, the MPD 8 is a DSD dac. DACs that up-calculate their signal to DSD, such as Playback Designs, EMM, the popular PS Audio dac, etc., soften and mush-up the sound to take away the dynamics and aliveness of the music. So in most systems today, which are highly inefficient box speakers, this lack of dynamics is a BIG negative, considering many of today's top DACs such as TotalDac and MSB are very musical with PCM. Obviously, system synergy is key, and the gentleman who mentioned this DAC, Chuck, runs his system with Boulder 3000 series amp, so Chuck is not starving for dynamics.
From a price standpoint, this is something like $25K, so there is not a large market for this, and whatever market there is, has converged to other brands (see more on bandwagon effects below)....
And how many dealers carry this product? What else do they carry? What brand do they favor/ feature in their top systems?
How many shows do Playback go to? And what systems do they partner with? MSB partners with Magico, to help draw those fans to expose them to MSB. Wislon is with dCS, and they always draw a large crowd...
I discussed the influence of "audio journalists" before, but again, people look to authority figures in a state of confusion...dCS had a huge reviewer blitz with their Bartok and Rossini DAcs , across many reviewers, in the last year, who didn't mention analytical tendencies of dCS is so many systems... But most reviewers just follow the herd , which further helps entrench the more successful brands....Does playback nurture relationships with audio journalists, especially open-minded ones?
Additionally, luxury and entertainment industries are bound to bandwagon effects... The bandwagon effect is where popular brands become even more popular as people gain knowledge of them from their friends, experienced influencers, dealers, online boards, show reports , “audio journalists “, etc. What this means is that people’s tastes converge on select few brands rather than be dispersed across the assortment of available offerings. We see this with popular music and movies all the time, and here we see MSB, dCS, TotalDac, and Lampizator as passionately discussed by the fans and owners and getting the sales in this upper price bracket (social proof influence force acting on us). There may be many excellent products out there, but they just don't get the oxygen.
I could go on and on, but the factors above - in combination - help answer your question of why some brands are popular while others suffer.