Same reason tech patents last for 20 years and not 5.
You've got big moneyed interests who benefit from long IP terms and small groups of people that might benefit from shorter IP periods. End result, no legislator will push for shorter terms, they just don't care about it.
Disney can show how much money they'll lose if the mouse becomes public domain (and how much that'd hurt campaign contributions) while Mickey's auto repair shop will struggle to show similar losses if they can't put the mouse on their billboard.
You've got big moneyed interests who benefit from long IP terms and small groups of people that might benefit from shorter IP periods. End result, no legislator will push for shorter terms, they just don't care about it.
Disney can show how much money they'll lose if the mouse becomes public domain (and how much that'd hurt campaign contributions) while Mickey's auto repair shop will struggle to show similar losses if they can't put the mouse on their billboard.