If your device is still sitting at Kit Kit (4.4) and you haven't heard of any updates or progress from your carrier about even a 5.x (Lollipop) update, then it's unlikely that you'll see them (the carrier) send-out a MarshMallow (6.x) update.
That being said, depending on your device and the development community for it, there may indeed be custom 6.x ROMs available for it.
The more popular and widely-used a device is, there larger the dev community is--although that is not to say that there are not many, many islands of enterprising groups of devs that work on the less well-known devices, too.