I think the best argument for why they did it that way is that the attack was essentially a test run. Earth was an unknown variable to them, so they wanted to see how effective the weapon was against the planet itself and how Earth's defenses would respond. They couldn't necessarily anticipate every variable involved in using the weapon against an unfamiliar world without actually testing it in the field.The question of Bush aside for a moment, I never understood why the Xindi were so stupid to start a test run of their weapon on Earth, thereby warning their clueless enemies about them with huge red letters. It doesn't make an awful lot of sense, rendering the entire season 3 premise implausible, imo.
If anything, the Xindi's mistake was underestimating what Earth would do once it knew it was under attack. They were confident enough in their technological superiority that they apparently didn't consider the possibility that this relatively primitive species would come looking for them.
I've never really understood this argument. A prequel doesn't mean everything has to be something we've already seen or heard about. The galaxy is huge, the fact that a species wasn't mentioned in another series doesn't mean they weren't there.And I have another huge issue with ENT season 3: The show was supposed to be a *prequel*. I don't want to see an entirely new major alien race as supervillain, one that was never a topic later in the timeline, in a prequel. I want to see how the world we know came into place -- exactly what they thankfully did in season 4.
The Andorians and Tellarites are a perfect example. Both appeared in TOS, and both are founding members of the Federation, yet TNG and DS9 almost completely ignored them. We barely saw an Andorian in TNG, and I don't think either species appeared in DS9. Obviously, their absence from those shows doesn't mean they didn't exist. It just means we weren't seeing every corner of the galaxy. The Xindi are no different.