As of the time of the series, Eri is working full-time, presumably at her own practice. Lawyers work notoriously long and stressful hours, and it's not unheard of that they abuse amphetamines just to make it through the day. While I highly doubt that last bit applies to Eri, taking care of Ran in the manner expected of a diligent mother is something that would've detracted immensely from her career. Therefore, she probably let Kogoro deal with that, seeing as he evidently had a lot more free time on his hands.
It should be noted that Kogoro and Eri are legally separated and not divorced.
(Also, there's a designated thread for asking questions. Next time you want to know something you should do this there instead of making an entirely new thread.)