I think the slow difficulty curve is good for people who only play games casually, but I don't think that makes up vast majority of this player audience. In any case, I think that this slow curve was the safer option so that more people would possibly enjoy it (edit: it as in the game) and for a longer period of time.
In the future, I think a good way to fix this is by allowing people to start at a certain level (like 30 or 60), and have the goal to be to get the highest score instead of the highest level, having people earn more score the higher the level they are at. Kind of like the system used in Tetris (or at least the browser version I play sometimes).
I get that there is a 48hr time limit, and you did a great job at making a game in that time, but for next time (or some small update, idk), this would be an easy solution that theoretically wouldn't take that long to implement.