For the same reason we think before we talk... You learn more if you keep your mouth shut when you are in an unfamiliar neck of the woods.

Some of the other comments are very good. I doubt if we had some real understanding of how everything really holds together we would be so dumb to change thing before we had made an intelligent assessments first. Here on earth we are losing species every day... why go and stuff up some other potential ecosystems before we understand them and potentially use them. They are "the locals"... scorched earth policies are always going to have a cost (insert your recent examples here).

For the slightly more intellectually challenged members... think of the red plague that Tom Cruise came across in "War of the Worlds".... not from the alien's point of view but the inhabitants of Earth's point of view. If 19th Century man can understand the problem (HG Wells) why is it so hard for 21st Century man to do likewise? I guess it is useless to tell people that Art mirrors Life (even after more than 100 years).
At any rate, whether we expect another invasion or not, our views of the human future must be greatly modified by these events. We have learned now that we cannot regard this planet as being fenced in and a secure abiding place for Man; we can never anticipate the unseen good or evil that may come upon us suddenly out of space. It may be that in the larger design of the universe this invasion from Mars is not without its ultimate benefit for men; it has robbed us of that serene confidence in the future which is the most fruitful source of decadence, the gifts to human science it has brought are enormous, and it has done much to promote the conception of the commonweal of mankind. It may be that across the immensity of space the Martians have watched the fate of these pioneers of theirs and learned their lesson, and that on the planet Venus they have found a securer settlement. Be that as it may, for many years yet there will certainly be no relaxation of the eager scrutiny of the Martian disk, and those fiery darts of the sky, the shooting stars, will bring with them as they fall an unavoidable apprehension to all the sons of men.

The broadening of men's views that has resulted can scarcely be exaggerated. Before the cylinder fell there was a general persuasion that through all the deep of space no life existed beyond the petty surface of our minute sphere. Now we see further. If the Martians can reach Venus, there is no reason to suppose that the thing is impossible for men, and when the slow cooling of the sun makes this earth uninhabitable, as at last it must do, it may be that the thread of life that has begun here will have streamed out and caught our sister planet within its toils.

Dim and wonderful is the vision I have conjured up in my mind of life spreading slowly from this little seed bed of the solar system throughout the inanimate vastness of sidereal space. But that is a remote dream. It may be, on the other hand, that the destruction of the Martians is only a reprieve. To them, and not to us, perhaps, is the future ordained.