Law vs. Chaos is an interesting concept - it reflects something about the nature of the universe which the Good vs. Evil dynamic doesn't, in my opinion. Good and Evil may or may not exist, depending on your worldview, but there's nothing particularly profound about the opposition between the two; it's essentially QED. Law vs. Chaos, on the other hand, hits on something deep and true about the lives of human beings.
That is, our lives as we live them are to all intents and purposes governed by Chaos. In the naivest terms, we never know, when we get up in the morning, what is going to happen that day; in the more complex sense, the world is governed by a multitude of random factors and quasi random factors (like the weather, which may not strictly be random because there is some method behind the system even if it is beyond our ken, but which is random in the only sense that matters - we can't really predict it) which make it impossible for us to adequately understand as a whole. This is true for the most banal situations (my train may be late tomorrow but I can never know in advance, and if nobody tells me why, it may as well have been because somebody rolled a dice and decided it should be so); it's also true for the most important ones (the economy, your own physical and mental health, earthquakes; we don't know why earthquakes happen when they do and we don't know why one smoker will develop lung cancer where another doesn't, so the reason may as well be randomness).
But the universe itself is a profoundly Lawful place; indeed it is the very essense of Lawfulness - this is why we talk about The Laws of Physics when we discuss the fundamental nature of things. Again, this is true in both very banal and very important senses. If I drop an apple and there is nothing to stop it, it will fall to the floor. If you add two to two you will get four. If you mix sodium and water you will get an explosion.
This dynamic - the interplay between Law and Chaos, Predictability and Randomness, Known and Unknown - also has more interesting philosophical dimensions than Good vs. Evil, because Law and Chaos are both neutral propositions. It doesn't make sense to say that Chaos is Bad, just because a sudden snowstorm delays your flight and makes you late for a meeting, because next year a sudden snowstorm might delay your flight and lead you to meet the love of your life. Similarly, it wouldn't make sense to curse the laws of physics and call them bad if somebody dropped an anvil from a sixth floor window onto your head - Damn you, Gravity! - because the laws of physics, indirectly, might be what save your life; knowing about them is what allowed human beings to construct the combustion engine and thus the ambulance that takes you to the hospital.
The idea that there might be Gods of Chaos and Gods of Law and that they are locked in conflict therefore doesn't strike me as a preposterous notion - it makes sense in my gut, because it reflects the nature of life as we know it. (To make it clear, I don't actually believe it to be the case; just, if somebody told me that people in Timbuktu did, I wouldn't find it counterintuitive.) There is such a thing as Law and there is such a thing as Chaos and they interact in all manner of ways, and it is a short step from there to the preposition that they are in dynamic conflict.
Good vs. Evil is a different kettle of fish. We know that we like Good better than Evil. More importantly, we also know that Evil is always what the other person is doing. I'm of the opinion that, objectively, some acts are Evil, but the perpetrators of those acts never think in those terms. The Final Solution didn't come about because the Nazis wanted to be Evil. It came about because the Nazis thought that Jewish people were Evil. The act itself was an Evil one, but it didn't arise from any diametric conflict between two fundamentally opposed philosophical schools - it wasn't about the Nazis fighting for the cause of Evil against Good. In their own twisted logic it was the opposite.
The idea of an explicit Good vs. Evil conflict in the world, then - that there are Gods of Good and Gods of Evil and that they battle for the cosmos - seems fundamentally ridiculous and unbelievable in a sense that Law vs. Chaos doesn't. Because what God, and what follower of a God, would explicitly campaign for the cause of "Evil"? It makes no sense; it's artificial; if you think about it for even a second it just doesn't seem like anything approaching a fit for how the world works and what motivates people.
Now, I can sense some of you forming an argument in your mind that goes something like this: "Good and Evil can still exist even if they don't have explicit proponents. Maybe Sauron doesn't believe he's Evil and thinks he's Good; that doesn't preclude the existence of Evil and a conflict between it and Good - Sauron is just mistaken." You're right of course. But I think you'll recognise my fundamental point, which is that followers of Law and Chaos (if they existed) would be able to announce, outright, that they are a Follower of Law or a Follower of Chaos in a way that makes sense, whereas nobody no matter how steeped in sin would ever be able to openly claim themselves a Follower of Evil and be believable as a person. This in my opinion makes Good vs. Evil a weak and artificial conflict of forces for a fantasy cosmology, and Law vs. Chaos a compelling and believable one.