All Topre boards featured on Massdrop have done well in sales. My guess is that Realforce boards are much broader in supply, and just easier for Massdrop to hit someone up to obtain.
Personally, I probably won't be buying the HHKB on Massdrop simply because the tax and shipping barely saves me much from MSRP. However, a good chunk of the keyboards I own from those who end up not liking their purchases from Massdrop. The Realforce 104UB-DK that I got off a Massdrop trade continues to be my favorite feeling Topre board. I own an 86U and an FC660C printed, but for some reason the 104UB-DK just has better resonance and a more solid bottoming-out experience (slightly snappier than printed FC660C and not as mushy as the 86U). The 87U 55g is real close to the 104UB-DK, but it's just too firm for me. I hope the 87U 45g EK edition is what seals the 45g deal for me if not a dyesub/blank FC660C. I'm still getting an HHKB though because I use my printed FC660C so much for its softness.