Your questions is vague in certain contexts. I am assuming what your question might be.
1) Why did not the aliens put the shield in the altered present?
Ans.- In altered timeline the shield does not exist. Remember what O replied in response to Js asking for the shield to stop the Boglodite invasion. She said:
No K. No defense system. So this option is out.
2) Why did not the aliens put the shield in the past(where J traveled back)?
There may be two reasons for that.
A) Boris changed the past resulting change in present where earth is, in the altered timeline, now vulnerable to the attack of the Boglodite, as the shield is not present there. Now in reality Agent J was the one who was responsible for covering the earth with the shield. So he must take the responsibility again to put the shield. Like Griff said:
The bitterest truth is better than the sweetest lies.
He has to go to Cape Canaveral.
He must deploy the Arc Net as he did before.
There's no other way.K is the only hope in saving Earth. Can I save him?
Is there any future where I save his life?
But where there is death......there will always be death.
So you see there was always J who was responsible(and experienced too) for putting up the shield.
Now this is an obvious logic behind the act. But I want to add another theory on my own.
B) We know that Bogoldite has the ability to destroy each planet and their inhabitants. Now if they requested any friendly alien species to do the job, there are several harmful possibilities which arise with this.
i) Threatened or bribed by Boris, the species may hand over the Shield to him.
ii) It may think to put up the shield on its own planet in lieu of the earth to survive from Bogoldite because at the end of the day that planet is their home.
iii) The most important reason is Earth is home of humans and not the aliens. So there may be lack of enough try from them which might cause in disaster.