I would guess that Half-Elf is a full race for two reasons: first, because keeping it that way suits the "flavor" of D&D, in the same way that nearly every Wizard (i.e. "Magic-User") throws Fireballs and second, because the use of templates has dramatically increased since 3.0 was originally developed. That said, I personally would prefer to see it as a template, since I most enjoy flexible character creation, so I love templates, multiclassing, feat choices, variant classes, etc. I chalk that up to personal preference (and years of playing HERO rules).