Complex just means (whatever) "we dont understand" but you asked WHY this observed did it infer, imply or induct Design"" yes?
Well I guess the first thing to be sure of is that this is NOT a univocal word for the science of complexity physics though I dont doubt that some creationists might ONLY be using it in that sense. Any way using only that use of the word would make the explanation I give below not sufficient even if necessary to the extant extent I consider.
One answer might be that they are able to conclude the sublime in that pattern and then THINK to a beauty that it is not and from there guess it IS THUS designed but I would have thought there is not this much sophistication but rather from a moral position instead ANY practical reason might grant that no matter how nature is traced the invention of it (in any human terms) could apirori have been and if it was then it will at least by will be by design should the thinker have a head on their shoulders. If you are asking why evos might not agreee with this thought process then I would need to go a different route with the wordings but the conclusion at least philosophical can have the same logic in the crass vulgar sense.
I hope that helps. Best, brad.