The reason why is, put simply, that they did a much better job at making the 3DS hard to hack than they did with the DS.
Because of a feature in the processor called "execute disable bit" it's impossible to use most of the previously normal ways to hack it and finding a good place (if many even exist) to use one of the other hacking methods that ISN'T blocked by that is what has been hard. They have finally found one BUT it's so easy for Nintendo to fix with a firmware update and destroy all their work in one shot that they haven't published it until they have something better to go off of.