Mat 13:29 But he said, No; lest while you gather up the weeds, you root up also the wheat with them.

At any early stage, you cannot tell the good from the bad. For Christ, the nurturing of the good is more important than destroying the bad. This is the whole purpose of the world, giving people an arena in which they can produce something of value.

In the Greek, it is interesting to not that they word translated here as "said" is a little different from the most common word used for that in Matthew. Usually, Matthew uses legô, which means "to recount" or "to tell." Here Christ uses the term phêmi, which has the more specific meaning of "to make known," and "to explain."