Question

why might a story have a longer rising action then a falling action ?

Answer 1

It could be the author is trying to extend the story, dragging out the tension. It could also be that there are a series of anti-climaxes which the falling action will not be subject to.

Answer 2

Rising action is used to complicate the main conflict and arrive at the climax. These complications are used to either delay the climax or prepare for it. (Preparing the character, the reader, the setting, or the opportunity for the climax)

Falling action is the same yet seemingly opposite thing for the conclusion.

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Instead of: Intro->Rising->Climax->Falling->Conclusion

Think of it this way:
Intro
Rising -> Climax
Falling -> Conclusion
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Recap Reasons:
1)Longer rising action can be used to arbitrarily prolong the story, if the story is profit based or similarly motivated by something non-literary.

2)Otherwise it is to further establish character or complete something needed for the climax - when the conclusion of that climax's conflict doesn't need as much preparation to be reach (falling action) after the climax occurs.

Answer 3

a longer rising action is much more intensifying

Answer 4

Style. If you are writing a love story, there is no specific reason to unbalence the equation. However, if you are writing an action novel, then it's practically a requirement to have a longer rise, so that when the fall comes, it's very quick and (hopefully) exciting. In an action story, you put the protaganist in a bad situation, and then progressively make it worse and worse until the climax. At this point all that is left is wrapping up loose ends. If you are done, be done. Otherwise setup for the sequel, then be done.