It could be because you're sleeping better. We remember our dreams when we are awakened frequently out of a dream state. Someone who knows better could correct me, but I understand that we only dream when we are in REM sleep. There is a deeper stage as well in which we don't dream. My hunch is that you are still dreaming when you are in REM, but you make it to the deep sleep stage now, which means you don't remember the dreams you had. Just my unprofessional opinion