Because the problems were found early, so they were easy to correct. So if you expect your developers to do a credible testing job, allocate the time for them to do it. Say thanks I have always been bewildered by managers who refuse to track errors back to the individual developers. Ironically, testers often know who the culprits are, but their own managers usually don't. If you really believe that it's cheaper and faster for developers to find problems than it is for testers or customers, then you should encourage them to do it.
Measurement is a great motivator.
Recognize and reward those who produce the cleanest code. Add quality objectives to their annual reviews. Create a culture that values quality. While I don't advocate the use of error counts to hold anyone up to scorn and ridicule, I do think they are excellent indicators of the need for training or discipline. So instead of complaining about coders, come to their rescue.