This is an interesting question worth looking into. Adults are forever lamenting the shortening attention spans of each generation. With angle changes in movies every three seconds and video games that are motion extravaganzas, it is hardly surprising that children are bored with the shockingly inactive letters on a page.
I've always felt that reading is a learned skill. Interest has to be nurtured in a child. Parents have to not only introduce reading but also show their children that they enjoy reading themselves. Bedtime stories can have a powerful impact on how the future adult views the solitary pastime of reading a book.
But what about slightly older children that already have formed views on reading? Short stories may be a gentle way to introduce them into the literary world. A short story is manageable. It's not a threatening 400 page novel with twists and turns everywhere. Due to the length of the short story, the language is usually concise and clear. It's also no difficult to keep track of all of the characters.
For me, my passion for reading did not really spark until I found a series of Star Wars books that peaked my interest back in the day. After that, I would read anything I could get my hands on. But it's important to have that first "spark." A short story could be something that provides that opportunity.