The phrase "necessitated by" implies that something is required due to a certain circumstance or situation. Synonyms for this phrase include "mandated by", "required by", "obligated by", and "compelled by". Other alternatives include "imposed by", "demanded by", "dictated by", and "enforced by". Essentially, all of these phrases convey the same meaning and indicate that something is necessary due to external factors. Using synonyms for "necessitated by" can help add variety to your writing and more accurately convey your intended message.