Here's why:
* Helpful is a one-syllable adjective.
* For most one-syllable adjectives, we add "-er" to form the comparative.
* However, "helpful" is considered a bit of an exception as the "-er" ending can sound awkward.
* Therefore, we use "more helpful" as the more common and natural comparative form.
Here are some examples:
* "She is more helpful than her brother."
* "This website is more helpful than the previous one."