It has a more restrictive number needed for it to come up than other finals (2-6 people), and we've reduced the frequency, so the compounding effect makes it ultra-more-rare. There are still other finals that could be picked if you have 2-6 people other than Royal Fumble so those are often chosen instead as it has a low likelihood of being rolled.
The reason we've reduced the frequency in general is because there's an exploit in that level which causes issues, and we haven't pushed the fix live yet, but are going to do so with S3 launch.