This decision was taken because following review, there were concerns about the levels of munitions in U.S. stockpiles. The U.S. is the world's most foremost fighting force and it's important to the Trump administration that it remains that way. Alternate ways of sending Ukraine the weaponry it needs are being provided to the president.
By reneging on weapons shipments that were previously approved by Congress, not only does this weaken Ukraine's defense when Russia is launching its biggest attacks of the war, but it also raises questions about whether the president is overstepping his authority. This is reminiscent of his decision to cancel weapons to Ukraine in 2019.
Ukraine is grateful to the U.S. for its years of support. However, by weakening Ukraine's defenses at this critical juncture, it does nothing to promote peace and only incentivizes Russia to continue its war and aggression.