Well, if you watch their videos most of what is caught is from the fishing industry, but it's probably a difference in durability. Plastic from rivers etc. are going to fairly quickly break down into (polluting) microplastics because the plastic is pretty thin, so it's best to stop that at the rivers, while a lot of what they are catching in the garbage patch is from the fishing industry because it's all made of thicker plastic and would take a lot longer to break down.