JST Matt 21:34 And again, hear another parable; for unto you that believe not, I speak in parables; that your unrighteousness may be rewarded unto you.
So it wasn't because parables are a higher form of teaching. It wasn't some means of protecting people who weren't ready to hear his message. It wasn't to reveal, it was to conceal. It was, according to Christ, to reward the unrighteousness of the wicked to themselves.