Why is it that my engagement is not being finalized despite suitable suitors proposing to me?

Allah decrees nothing except for a reason. The delay in marriage might be a manifestation of Allah's love, so He may hear your supplications, or a trial to test your patience, for which one is greatly rewarded, or a diversion from an evil that Allah knows. Therefore, one must have a good opinion of Allah, supplicate to Him, take the necessary means, and perform Istikhara.