It's 5 because we once arbitrarily made it default to 5 and most people tend to be fine with that. That said, you can just open powder.pref, find UndoHistoryLimit and crank it up to like, say, 100. Note though that every level of undo basically saves a copy of the entire simulation, so having a long undo history can become very memory-intensive. I set it to 100 and easily managed to get to 2 gigabytes of memory allocated to TPT.