It appears that MTD file systems are designed for small systems. I don't know about UBIFS in particular, but typically they work well only on small devices.
To go and write your own file system for MTD that would scale up like XFS or Ext4 can do would require years of development and for Windows this is a virtual impossibility unless you can get Microsoft on board. Meanwhile people have been writing block storage to memory device translation for decades. People have been doing these translation firmwares for a very long time.
Another thing is that compatibility with existing interfaces is important. They want to have the ability for people to purchase and use the devices with the minimal amount of effort.