Kerrigan not so "Queen ***** of the Universe" after all.

She was emotionally a wreck. She became aware of "a threat" in the epilogue to Brood War (presumably the hybrids, though I doubt she had the name at the time). It wasn't until the Dark Templar Saga that she regained her confidence and was willing to fight.

And even then, she obviously didn't get it all back, as she wanted to give up, unlike Zeratul.

I think she made her discovery right before the Second Great War, and began her invasion immediately thereafter. (So emotional weakness plus information.)

The terrans knew about the artifacts more than Kerrigan because Narud (Duran?) told them where to find them. Kerrigan wanted to prevent the artifacts from being blasted. Without info, she was forced to simply blanket any planet where there was a chance an artifact was on. The fact that some of these planets (eg Mar Sara, Tyrador VIII) were Dominion planets was a bonus.

 

Note that kerrigan isn't actually that good at controlling the zerg. She can pacify the overall hive, but only control a segment of it at once. It doesn't really hamper with conquering individual worlds, but she'd have trouble executing either an overall attack or defence. Or at least, that's whats implied in the dark templar saga.

 

I guess that would explain how the dominion invasion didn't get completely decimated by the 6 billion zerg that live there.

Then again, most of those zerg were likely on the offense, but still willing to bet about 1 billion zerg lived there since Char was Kerrigan's flagworld.