The addition of parameter names to the class file format is covered by JEP 118, which was delivered in Java 8. There was a little bit of discussion about why inclusion of parameter names was made optional in OpenJDK email threads here and here. Briefly, the stated reasons to make parameter names optional are concerns about class file size, compatibility surface, and exposure of sensitive information.
The issue of compatibility surface deserves some additional discussion. One of the threads linked above says that changing a parameter name is a binary compatible change. This is true, but only in the strict context of the JVM’s notion of binary compatibility. That is, changing a parameter name of a method will never change whether or not that method can be linked by the JVM. But the statement doesn’t hold for compatibility in general.
Historically, parameter names have been treated like local variable names. (They are, after all, local in scope.) You could change them at will and nothing outside the method will be affected. But if you enable reflective access to parameter names, suddenly you can’t change a name without thinking about what other parts of the program might be using it. Worse, there’s nothing that can tell you unless you have strict test cases for all uses of parameter names, or you have a really good static analyzer that can find these cases (I’m not aware of one).
The comments linked to a question about using Jackson (a JSON processing library) which has a feature that maps method parameter names to JSON property names. This may be quite convenient, but it also means that if you change a parameter name, JSON binding might break. Worse, if the program is generating JSON structures based on Java method parameter names, then changing a method parameter name might silently change a data format or wire protocol. Clearly, in such an environment, using this feature reliably means that you have to have very good tests, plus comments sprinkled around the code indicating what parameter names mustn’t be changed.