When you have access to many of the same, you can get proficient at darting them as you know their routine. Imagine you needed Sucho but only saw it now and again, you would get much less DNA per dart and be searching them out. Think Rex and how hard to dart it is.
We first had this issue with Euoplocephalus. They were everywhere to the point people wanted a filter to remove them from the screen. Update comes out with Tragodist and Euoplo all but disappears from everywhere. Folks came back on complaining that they never find them now.
I dart everything I come across, unless I am maxed out on DNA for it. I was able to create and level up some of the latest hybrids with DNA I had in storage. Some that I have over 150k of I may never use, but then again, I may. I believe that at some time in the game all DNA will eventually get a hybrid. Might not be any good, but may turn out to be surprisingly good. I am going to make hay while the sun shines.