Reason is because is incorrect.

The meaning of because is for the reason that, and the meaning of reason is a basis or cause, so using these together makes a redundancy.

The reason that our campaign is gaining in popularity is because people like our platform means The reason that our campaign is gaining in popularity is for the reason that people like our platform.

We ask the reason is what, not the reason is why. We would never say Why is the reason; we would say What is the reason.

The word is is a form of the linking verb to be. Linking verbs most often are followed by adjectives, not adverbs. Adjectives answer the question what. The word that can be used to introduce an adjectival clause.

Adverbs answer the question why. The word because can be used to introduce an adverbial clause. Since adjectival clauses, not adverbial clauses, usually follow linking verbs, the word because cannot be used here.

The correct construction is The reason that our campaign is gaining in popularity is that people like our platform. (You could also re-write the sentence to Our campaign is gaining in popularity because people like our platform.)

Sherry

Sherry’s Grammar List and Paul’s book–Literally, the Best Language Book Ever