Historically, very few colleges admitted women, thus when they began doing so, they became "coeducational" institutions, but the prominent feature of such institutions was the (then minority) presence of women, who thus became associated with the term. There's little in history or language that is symmetrical or equitable - although, these days, the smart money on the unspecified college student is that she's a woman.

PS: I swear when I wrote this there weren’t 4 other perfectly good replies visible yet. Apologies for the redundancy