Beyond actual capabilities of log4j vs JDK logging, it's not clear that anyone considered how adoption would work. At the time, log4j supported several back versions of the JDK, while JDK logging obviously required you to be on the newest version, v1.4. As a result, if a library wanted to support any older JDKs, they couldn't switch to JDK logging, or had to support two parallel logging frameworks for little gain. And if your libraries are sticking with log4j, what's the advantage to your program of using the JDK logging?
slf4j is the current solution for this kind of problem, a common interface to various different logging solutions.