Main-Class isn't part of the OSGi bundle standard, and I don't believe that maven-bundle-plugin recognizes it.
You can follow the instructions for using an existing MANIFEST.MF file and add the instruction
<_include>src/main/resources/META-INF/MANIFEST.MF</_include>
and then include your Main-Class
directive in that file. This is a little clunky, which may suggest that you're using the wrong tool for the job. If you just need an executable jar file, there are other Maven plugins that might be more suitable, like the maven-jar-plugin.