Partly it is budget issues for any country. It certainly has been for the US. US war plans saw manpads as a low priority item, and if it were the US fighting this war this would most likely be a correct assessment. So why develop something new?
In the case of Ukraine since 2022, they have been getting Stingers for free, and because Stingers were available in large numbers, not because they are ideal. Ukraine isn’t doing cost-benefit analysis, they are just asking for what they can get.
Ukraine’s original manpad was the ex-Soviet Igla. But they ran out. So they have been supplied with a zoo of western and ex-Soviet manpads, not just Stingers. Polish Piorun, British Starstreak, more Iglas from various sources, even ancient Soviet Strela. Etc.
Interesting - thanks! I guess anything beats a video I saw that was purported to be a Ukrainian soldier shooting at an airplane with a bunch (six or so) of AK-47s bolted on to a tripod, with a strap around the triggers, and he would pull the strap and let loose all of them as he tried to hit the plane! (He wasn’t leading the plane at all!)