Because nobody cares about shootings in like 2/3 of the world and crimes in most countries don't make it to the news, second, it's about definitions.
So you have 600 mass shootings in the US, but at the same time you have only 11 with and with more than 3 fatalities, in Russia for a mass shooting to qualify as it you must have 6 killed, and in some countries, it's even worse as they don't even count and mark each as a homicide.
Plus, there is the financial aspect, in most poor countries it's too hard to try and buy a gun and ammo for it, you have to brie people, and you don't have the pleasure to try to go and train yourself, so harder for people that never had a gun in their hand to think like that, they use knives like the recent attacks in China or simply use a car to commit such things.