Honestly, my inclination is a fusion of two questions:
[X] Why us, and why now?
Please understand - this is not so immediate as "why humans" and "why after 3rd Battle of Sol"... The answers to those would be "because Practice" and "no more time left to waste", as my inner cynic tells me. No, this is a lot more encompassing.
It is a question about why didn't they choose any champions before, or at least - none that we know of.
It is a question about what makes all those who could have been their champions in the eons past, less suitable than humans now.
It is a question about whether there were any other champion candidates, others with Practice or similar abilities.
It is a question about what was it that truly changed, what brought it all together that they HAVE to act now, in this era*, when they didn't before.
A question about both the past of the galaxy, and of its current state.
*I acknowledge that humanity may not have been the most pertinent factor, even. For all we know, suitable champions have risen in the past, but they didn't have any good allies, like we would now, to be able to bridge the gap between them and Shiplords, and that which lies beyond Shiplords.