The main reason that grant proposal reviewers with a conflict of interest should remove themselves from the review process is because:

The correct answer would be, It lessens the chance that bias will affect the review process.

Explanation:

Grant proposal reviewers are people who review the proposal came to them to approve or reject a grant. They select the proposal on the basis of certain ground to approve the grant. If there comes a proposal, which conflicts with their interest, then it is better to get themselves removed from the review process, because if they do so, there are pretty fair chances that they become biased and can reject the grant on the basis of their own views. So if they get themselves separated from the review process, it would lessen the chance that bias will affect the review process.