The fact that a semi-dormant artery runs through our forearms clearly indicates that there was once a need for this additional blood flow. Perhaps this artery was initially formed in a time when enhanced dexterity was essential for survival. For modern times the dexterity needed for keyboarding and thumbing cell phone text (or even penmanship - still) has excited the artery's control mechanisms. This could explain the new prominence, but how many 200,000+ year-old cadavers does science have to work with? And the concept of a dire need for enhanced dexterity is also worth some thought.
There is also the possibility that the artery was more functional in the not so distant past, when manual labour - particularly in largely agrarian societies - was more widespread. Manual labour has become less prominent in Western societies with the advent of the Industrial Revolution, and the subsequent development of steam and electrical energy, closely followed by the introduction of internal combustion engines enabled by the vast amount of energy released in hydrocarbon exploitation. This more recent de-emphasis on manual labour isn't uniformly global, so, as other posters have pointed out, a wider sample size might find that other societies have contnued to have a functional mid-forearm artery.