Why might an endorser want to limit the term of an endorsement contract with a company?

for better opportunities

Explanation:

Endorsers are usually in exclusive contract with companies that forbids them to work with other competitors. Celebrities have their high or low times, when they are at their peak popularity, they would want to be associated with the best and top of the line brands and vice versa. They prefer to switch according to the situation that's why they try to limit the term of contract.