It's something we discussed in quite some detail. It felt as though some players found it too easy unless there was a complete defeat.
In all honesty it was really hard to find the right balance, as some people said it was too easy and others too hard!
We also considered having difficulty levels per stage, but it was extra work we didn't have time to do unfortunately. Now that we've got the game working more or less, this would be pretty easy to incorporate.