I suspect the reason for few users is because it's relatively simple to build such an app and so you have tons of competitors, such as Pleco. For example, the visualization you have is pretty similar to a learning tool that annotates subtitles I built in 2013 - it's no longer online but you can see screenshots at https://dspace.mit.edu/bitstream/handle/1721.1/90411/Miller_... and a video at https://www.youtube.com/watch?v=3j-eXUB3eaA