Science has always tried to find the "Laws of Nature." What that means is that we are looking for consistency among our observations. They are not just different things happening. There is something consistent going on that makes them happen.
Scientific theory is guided by "Occam's Razor," the principle that "one should not multiply hypotheses." Why? Because each additional hypothesis is an inconsistency, an exception. The fewer independent hypotheses you have, the more consistent is your theory.
That's what makes Newton's theory of gravitation so wonderful. It says that apples falling and the planets revolving in their orbits are really the same thing going on. And that's what makes the Ptolemaic theory of planetary orbits so bad, because it starts out with the idea that the orbits are circular and they lays circles upon circles upon circles ("epicycles") to account for the deviation from circularity.
"Intelligent design" adds to the Laws of Nature the proposition that there is a designer. That's not only an additional hypothesis, but it raises questions that could need yet more hypotheses to answer them. Who (or what) is the designer? Where is the designer? How does the designer implement the designs?
Darwinian theory, on the other hand, is consistent without any added hypotheses. Darwin himself understood natural selection, and now we know how variation takes place and how it's inherited.
Yes, there are gaps in the theory of evolution. There are structures in living organisms that are so complex that we don't know how they evolved. But if we trust that they did evolve, we have some hope of finding out how they evolved. If we assume they were designed by an "intelligent designer," we would have to penetrate that designer's mind to learn how they came about and why they have their present forms.