Must be part of it, yea! Same thing for me. I've had one lonely acceptance through Submittable in several years, but plenty when you email the journal directly. We use Submittable for Red Ogre Review - I don't think it makes our acceptance rate any different - but the platform is a bit impersonal compared to direct emails with authors. And makes it easy to blast out tons of subs at once, on the other side. Volume + impersonal vibe = easier to reject pieces?