It is a bit of a hold-over and a refinement from earlier editions where bards had to be wandering minstrel-types it was viewed that a Lawful character would form too many attachments to fit that archetype.
In second edition, the Bard had to be partially Neutral, just like the 3.x druid. Amusingly enough, The Complete Bard’s Handbook had a Skald kit*. Still had to be partially Neutral, though. But Lawful Neutral was actually still on the table.
*That book probably had the best kit system out of any of the supplemental books of that era. Instead of crappy, don’t-make-much-difference minor bonus and penalty, they actually outright replaced standard class abilities with new, flavorful abilities, just like 3.5 Alternate Class Features.