The MI-12 was wider than the wingspan of a B747. It therefore needed a very large airport to operate from and very strong taxiways to support it.
It was so large that smaller fixed wing were more versatile. It became pointless as it was not commercially or militarily viable to use a rotary wing to do a fixed wing job. As a technical exercise it was an amazing project but that was about all.
Think about trying to refuel such a large aircraft from small bowsers, or even trying to supply enough of a load to make it worthwhile to operate it. Simply too big for its own good.