Interesting game! I like the concept. For brief periods I got into the zone where I was nailing them back to back, but most of the time I felt overwhelmed. Imagine Lucille Ball in the candy factory. I think what you really want is to bring the player into the zone, where they are really hitting one after another. Its currently really hard to get there, even on the easiest difficulty.
Have you considered removing the difficulty choice up front, and adding a dynamic difficulty? All you need to do is vary the frequency of drops. When they hit one, the speed increases a tiny bit. When they miss one, the speed goes down a tiny bit.
There is no clear failure state. You just keep dropping balls and eventually it's over. Tetris has a perfectly designed failure state. You always knew how close you were to losing. In this, it's not clear.