Women and shorts vs. capris is probably not as much 'fashion' as just 'less hassle while out running'. Women running past groups of guys are routinely subjected to comments on how they look, how sweaty they are, how fat they are, how much their tits are jiggling, how small their tits are if they aren't jiggling, etc etc etc. Some women who would be perfectly happy to wear short shorts will cover up when running to avoid it.
P.S. I've never seen a bloke running in capri pants yet. Actually thought they weren't available for guys. I usually chuckle a little at the guys who wear full length tights then cover up with baggy shorts over the top as they're embarrassed to be seen in lycra. It usually looks rather uncomfortable...