I'm not a tech, but I would guess the same thing is stopping someone today that stopped someone up until today. And also the same thing that stopped all the groupon clones from becoming groupon and all the
facebook/twitter clones from becoming facebook/twitter. In the words of the once-famous Jerry Seinfeld,
No Resources, No Skills, No Talent, No Ability...
A very good point, SD did get its name out there and established

Care to elaborate what mistake was made with the maths.
First, I am not knocking the website/business. I think it was clever, well executed, and they deserve credit.
Comparing them to facebook is ludicrous when trying to infer they have a wide moat around their business model
that is impenetrable from others looking to take a piece of the pie.
As soon as a clone is polished up, offers better payouts for the same exact bet, SD will have to match it
(thus reducing profits) or admit they cannot control the market for this type of gambling. Word of mouth is
strong in bitcoin land and most users know where to get the best bang for their btc. A simple add on this website
and a few others would reach many users over a one week period getting the clone's name out there immediately.