First, "is this the reason" is perhaps less common than "is that the reason". It depends on the context of the conversation. I invite you to provide a longer dialogue so that we can discuss the context of the question. I would use "is that the reason" if only one reason has been brought up. If, on the other hand, several reasons have been brought up, either "is this the reason" or "is that the reason" would work. I know there's no obvious rationale to this but that's my sense of how native speakers would speak. It seems you are asking whether "why" is necessary. My answer to that is "it's neither necessary nor is it wrong to include it". Is that reason (why) you came here? Is that reason (why) you don't come over? Is that reason (why) you came here?

I would just say "Is this why you come/don't come here?"