Fortunately classics have the answer. Lexicon is from the Greek, and thus technically doesn't play by latin rules. The thing is, Greek neuter plurals aren't really plural; they go singular in agreement, and are effectively singular in every way (including verbs they take). Therefore to call the plural of lexicon lexica would be

-wrong
-blatant disregard for the language it came from
-blindly attempting to enforce latin on a Greek word

In fact, as Lexicon is neuter, the plural is lexicon, like sheep and sheep. As English probably doesn't like that, we just shove an 's' on the end.