Dave C's assessment of the complexity of adding difficulty levels is spot on.
Where do you make it easier? The match engine? Training? Transfers? Morale?
Let's say the match engine. This contains thousands of individual calculations, so for every single one you have to alter the AI. It's taken 4 years or more to get one difficulty level right.
Training is similarly affected - there are many things that affect the calculations (training schemes, coaches stats, player determination, age). If young players get better quicker, you have to make sure old players still get worse, otherwise you upset the balance of the game and teams end up with an average age of 35!
Each section of the game has similar consequences to changes. Not forgetting that they have knock on effects to each other. The last 3 or 4 years of debugging would be worthless.