That's because you probably bought the phone from a re-seller who re-flashed a "custom global ROM" (compiled by who knows who) which does not receive OTA updates because it's not compatible with the official releases. This is what happened to me. After noticing that I don't get the updates for a while I started to investigate and I re-flashed one of the official ROMs from http://en.miui.com/download.html.
They do security updates for Redmi 1S regularly though afaik. They also update their phones with the new version of MIUI. MIUI 8 is coming for Redmi 1S next month. [1]