Here's why:
* Comparative and superlative forms are used to express degrees of something. For example, "big" becomes "bigger" and "biggest."
* "Contended" describes a state of being disputed, not a level of dispute.
If you want to express different degrees of dispute, you would need to use different words or phrases, such as:
* More heavily contested
* Highly contested
* Most fiercely contested
Let me know if you have any other words you'd like to explore!