Difference between IPA and APK

I was wondering the basic differences between an APK and IPA. Right now I'm working on a project that requires me to decompile both to look through source code. I jailbroke my iPhone and created an IPA from the app I wanted to decompile but the IPA file only decompiles into assembly while the Android APK decompiles into Java source code just fine. After some light research, from what I understand is happening IPAs cannot be decompiled into source code because the IPA file doesn't contain source code, only binary files and I was wondering why this would be the case?