The Xindi were not in the business of annihilating whole planets for the fun of it, they were planning on destroying Earth because the Sphere Makers told them that—if they didn't—Earth would do the same to them in the future.
So they spent a lot of time developing the weapon, which needed to be tested, and they needed intelligence on Earth. Testing the destructive power of the weapon on any planet with life and sending a probe that would likely to be caught wouldn't have been prudent.
But killing two birds with one stone by sending a probe with the prototype weapon and testing it on the planet they hoped to destroy anyway makes sense, especially if you operate under the assumption that there would've been no way for Earth to know it was you.
That is, in "The Xindi", the council is shocked to find out that Enterprise was looking for them in the expanse:
Xindi-Humanoid: It could simply be a coincidence.
Xindi-Reptilian: You're being naive. Their planet is fifty light years away. It is not a coincidence.
Xindi-Humanoid: How do you know your contacts gave you accurate information?
Xindi-Insectoid: It's the beginning of an invasion. Hundreds of Earth ships will follow.
Xindi-Humanoid: They have no way of knowing that we launched the probe.
Of course, the only way Earth knew who perpetrated the attack was because Future Guy told Archer about it in "The Expanse".
There's also the matter of the different philosophies amongst the Xindi species. The Humanoids like Degra wanted to proceed slowly, but the Reptilians and the Insectoids wanted to attack Earth as soon as possible. They were constantly giving the other races ultimata to finish the weapon quickly lest the resort to other tactics. For example, in "Rajiin", the Reptilians threaten to use bioweapons when Degra talks of the delays in the project:
Xindi-Reptilian: And what of your work, Degra?
Degra: I haven't been able to assess how much data was lost. This will obviously delay us.
Xindi-Insectoid: How long?
Degra: Months perhaps. I'm not certain.
Xindi-Insectoid: The humans have been through our space!
Xindi-Reptilian: They attacked a trellium mine. We cannot wait any longer.
Degra: Think what you're asking me to do. Build a weapon to destroy an entire world. Do you understand how difficult, how dangerous this project is?
Xindi-Reptilian: Are you saying you're not up to the task?
Xindi-Aquatic: Dissension will solve nothing. We knew there could be setbacks.
Xindi-Arboreal: I agree. We are too far along to change our plan.
Xindi-Humanoid: Besides, we have no alternative.
Xindi-Reptilian: You know that isn't true.
Xindi-Humanoid: That option has been discussed and rejected.
Xindi-Insectoid: Not by all of us.
Xindi-Reptilian: Perhaps it is time to reconsider our colleagues' proposal.
Xindi-Humanoid: We do not have enough information about the humans!
Xindi-Reptilian: That can be remedied.
Xindi-Aquatic: It's too dangerous.
Xindi-Arboreal: I agree. Degra deserves more time.
Xindi-Aquatic: But if we do not see some progress soon, we'll have no choice but to consider all our options.
And we learn in "Azati Prime" that it was a Reptilian who manned the probe that attacked Earth: it's not inconceivable it was the Repitilians who pushed for a "live fire" test ahead of the finished product.