FWIW, it's not about the note being harder for forge, but people are less familiar with handling them because they are so rarely used, therefore people are less likely to pick up on the clues that it's a fake. A point that's largely moot now considering how few people use cash for anything any more.

Why not do what other countries do and just make the denominations physically larger as the value increases? Now it’s harder to wash and reprint forgeries. Also if you use different color paper for each denomination you have another way to prevent forgeries of the washing and reprinting kind.