Because for everything in the “varmint” class (say, coyote size and down), various flavors of .22 work fine, are easier recoiling if you’re going to be shooting a lot (like on a prairie dog field), are cheaper component wise, and already have a big head start in terms of barrel availability (probably a bigger deal back in the day). And for “big game” (say pronghorn and up, but really mostly starting with deer), most .25’s are considered kind of marginal. Sure, .243 Win is even smaller and has taken a lot of first deer, but most people eventually move to a larger “adult” chambering, even if it’s just 7-08.