It's the possible mnemonics and days to chect that does it for me, getting it solve in 30 hours is great but imagine the possibilities of checking for more than a week?
The longer they need to check, the less likely the prize is theirs, the more likely the cost is no longer worth it.
It’s neat for a set contest like this, but in practicality this won’t crack your average user’s private key before we all die.