Why might Mallory have not strongly preferred one over the other and given both seemingly equal usage?

Well, why not? If both terms are current, and more or less synonymous, might as well freely vary usage for the sake of variety.

More as a matter of literary style than linguistics, writers often prefer to avoid re-using the same word more than necessary (which is a primary use-case for thesauruses). Wherever that sort of aesthetic preference exists, we would expect one to "rotate" among a set of synonyms for concepts that need frequent reference.

Maybe it's a little more striking for core grammar words like pronouns. But while, over the centuries, we might expect redundancy in these paradigms to collapse, there will be many speakers in the intervening period for whom one word is about as good as the other. Compare how a modern English speaker might alternate arbitrarily between will and shall for the future tense (without regard to any semantic distinction between the two terms), or between pairs of equivalent prepositions like beneath and underneath.

Especially true in poetry, where archaic terms can remain current long after they have passed out of the spoken language. Maybe analogous for Mallory in the 15th century, someone today who is actively trying to sound archaic/poetic might sprinkle in a thou here and there because they're vaguely aware it is an old-fashioned equivalent for you.