I'd assume it's just a certain elo number threshold, which nobody has achieved yet. It's probably very high, maybe too high, because most of top 10 is only plat (Snook and Gent at the time of this post actually)/gold with hundreds of games already and a very high W/L ratio. It's just a matter of the top percentage of players continuing upward in elo number until Diamond is hit. But if only top 3 will ever get to Diamond the whole point of the rank is kinda redundant. I had the assumption top 50 or so could hold it and maintain it, but it doesn't seem like it'll turn out that way.