There are many many reasons not to do that, which your cunt size brain simply can't comprehend. Here are some of the few
1) 95% of the Ukrainian infantry use AK derivatives all chambered for 7.62 x 39mm calibre which is incompatible with SAR-21 5.56mm Nato standard. SO even if they bought it from us, it would be useless because they cannot find the ammo for it
2) Its a bullpup and most armies in the world will not use a bullpup
3) Its an expensive weapon. If the Ukrainians want to buy a 5.56mm rifle, there are many cheaper ones out there. And right now they need quantity over expensiveness. eg they can buy a chinese made copy of the M16 chambered for 5.56mm by NORINCO for 1/3 the price of a SAR-21