Might due to avoid being able to run away scot free. Imagine a map that is filled with enemies every 2 meters. If there was a diminishing return on heavy, you will be heavied for the first 30-40 seconds and be able to outrun any mobs after that without any consequences whatsoever.

It is annoying I agree, but the mobs themselves don't pose a threat to anyone anyway so either just keep running or kill them real quick. I'd wish for it to have a diminishing return too, well, only if enemies are actually poweful enough to kill me