Because it sounded the same on the galaxy sound system (the one jj2 uses, not plus), and you got a smaller filesize. Also, it was a way to make custom boss tracks.

There's a j2b converter. But please, don't use it.

J2b doesn't support several module effects, so converting modules to j2b is usually a terrible idea. I convert to mo3 (which is smaller than j2b, and doesn't screw up the track) if the module hasn't yet been used on j2o, and is a plus only level.

Anyone who finds a converted j2b should try to find the original. The unconverted JJ2 tracks were from the OEM versions, so the modules were probably betas, and some samples were improved for the retail release. Which is likely why the non j2b versions of the jj2 modules sound worse.