Technically the US answered the call for more guns sent by Ukraine by shipping them M777.
As I pointed out, without the firing system or guided munitions a M777 is more or less the same thing as a MSTA-B. Well, the M777 is 155mm and the MSTA-B 152mm, but I suppose the effect will be the same for whoever is on the receiving end.
It seems that, in the end, the Ukrainian army does spend its equipment pretty fast. Early on during the war some NATO official talked about Ukraine going through its stock of donated weapons at an alarming speed. Losses suffered by Ukraine are also relatively high, so maybe risking a "run of the hill" plain M777 is less of a dramatic loss than an upgraded one. The US are probably not to keen on seeing their stuff getting captured by Russia either.
In the end it is still a 155mm field artillery gun. Just like the T72 donated by Poland, though objectively useless as front line units, are still "armored mobile 125mm gun platforms". It's not much, but it is still something and better than nothing.
When/if the Pzh2000 ever get delivered to Ukraine now that will be more interesting.