When Kes left Voyager in the episode "The Gift", she used her psychic abilities to propel Voyager through Borg space almost instantly, cutting ten years off their journey. But why (in-universe) didn't she send them all the way back to Earth? (Out of universe, it would have meant the series was over.)
I think the episode deals with this relatively well. She gives them a gift while she's still corporeal enough to do so, throwing them as far as she can, while respecting the physical integrity of the ship.
PARIS: This can't be right. Our speed is - it's impossible!
KIM: We're coming apart!
PARIS: We've just dropped out of whatever it was we were in.
Had she tried to chuck them another 10,000 light years, the ship would presumably have exploded or fallen apart, killing everyone on board. And I think we can all agree that that wouldn't be much of a present.
We could postulate some explanations, but we don't know the answers. It's largely a mystery.
She missed. She is very new to her abilities. This reminds me of myself playing pool. The ball hardly goes the direction I want, let alone at the perfect speed.
She was not powerful enough. She is young for an evolved being. Maybe toddlers are too limited.
The word omnipotent is used to express that we cannot fathom a being's power or abilities; it is nonetheless a misnomer. Even Q admits that his powers have limitations. We can't begin to understand his abilities, let alone understand how he is limited.
Let's pretend we're human beings trying to build an ant tunnel. This job would be "beneath us" and quite "too small" for our fat fingers. We would need to create tools so we could work small and also not push too hard. Similarly, Kes may not have had the tools to be properly accurate. Anyone who has played paper football can relate to how hard it is to be accurate with a flick of a finger.
Maybe if she flung them all the way home, the Borg would mistake the trip as powered by Starfleet technology. The borg would see this as both a threat and an awesome new technology to acquire, so they would invade and destroy their home. She couldn't let that happen.
She knows that her role was to help so much and no farther.
I like @onepound's thought: she was being cautious. She didn't want to blow up the ship by "smacking" them too hard.
Hey, powerful beings have things to do, you know? Maybe she needed to go meet the other ascended Ocampa who were calling to her. She didn't have time to do it right.
While Kes certainly is not beholden to the prime directive, she does understand the idea that you can do harm by helping too much. See also #4.
As for my thinking, it clear she could have picked up the ship and walked it all the way home safely. So #4, #5, #7, and #8 seem most likely to me.
Although, I do find the question hollow. As the OP mentioned, ultimately the answer is the "writers didn't want to."
My take was that she sent them as far as her powers would allow her to, safely anyway. She had JUST gained them, so she may not have been fully able to control them, or even know their extent. That was the way I perceived it.