Wouldn't it be nice if we could make ourselves pure simply by eating the right things? Granted, it is hard to eat the right things. But it's a lot harder for most of us to say and do the right things.
We have all sorts of problems with our speech: we say too much, we gossip, we lie, we take the Lord's name in vain, we exaggerate, we distort, we slander, we wound, we curse, we are crude .... The list could go on and on because the tongue can get away from us. Part of the problem is that our mouths often let fly what is unredeemed in our hearts. Another part of the problem is that we don't think about what we're going to say before we speak it. Still another part of the problem is that we do think what we would say to wound, hurt, or get back at someone; then, when we are given the opportunity, we say it.
Purity is an ongoing struggle. Jesus is saying don't make it so simple, so external, as just a matter of what you eat. Externals may be nice for traditions, but they're awfully empty at redemption.