When writing, there are many ways to express the phrase "the reason is." One synonym for this term could be "because," which is a simple and direct way to explain the reasoning behind something. Other options include "due to," "owing to," and "as a result of," all of which convey a cause-and-effect relationship. If you want to emphasize the explanation, "on account of" or "in view of" would work. Another possibility is "since," which suggests a connection between two events. Using varied language not only improves the clarity of your writing, but it also adds interest and depth to your work.