It is based on luck, although it is pretty frustrating when you see how other ppl have gotten more luck than you do
I still haven't gotten any of the original rares (the eagle, fairy or wild) much less any of the newer ones.
I think the most frustrating and boring part is how long some of the breeding times for the common ones are. If you are going to give me a bunch of commons (like islands and poisons) don't make them be 20 hrs+ because as frustrating as it is to get them when I am trying for something else, the most frustrating thing is to wait for those long breeding/hatching times.
In fact, after the 7th (currently breeding) poison dragon, I have not been playing as often as I did just because I have to wait on that icky dragon to be done. My
suggestion, make the breeding times of the common ones to be shorter and the one for the rare ones (that I do not mind waiting for) shorter.