Short answer: tools do the same thing, but in different ways. The difference is how you can use them to create your project. Apache Maven adheres to the principle of "follow our rules", and Gradle gives you flexibility. Some developers do not want a new tool or do not have the time to learn how to use it correctly.
Now the long answer ...
To correctly understand why some developers are connected to Apache Maven, we need to take a look at past years.
Apache Maven is the same build automation tool as Gradle .
Maven was launched in 2004. The first version of Gradle was released in 2007, but in the early days it did not become as popular as Maven . Some developers do not like to learn something new, and most companies do not want to risk exchanging an already launched and mastered tool ( Maven ) for a new child in the block.
Gradle became popular when Android development rose. An Android project differs in design from Java EE / Web projects. Trying to use Maven on an Android project just seems unnatural; the tool was not ready to provide flexibility.
Until now, Apache Maven could be used in simple Java EE projects without worrying about ass, if you are already familiar with coercive conventions. Therefore, developers who are not aware of Gradle / have not contacted Android applications do not have the motivation to switch to another tool.
In my opinion, Gradle better than Apache Maven in every way you could imagine. This gives you flexibility and tries not to bother you, a “feature” that you cannot find in Apache Maven . If you don't follow the Apache Maven life cycle, your build will fail, that’s it.
For example, your code can be distributed in many directories in any form, if you use Gradle . If you use Maven and do not abide by the “agreement”, you will lose several hours modifying your pom ( Maven assembly file) to be able to understand and process the structure of your folder.
For example: Java source code should be in the src/main/java folder. If you have an old project with src/java structure, sorry, you have no guarantee that all Maven plugins will work properly.
Gradle provides us with a service and puts more features for comparison in a very organized schedule. Take a look. https://gradle.org/maven_vs_gradle/