Pretty Simple really longer videos
Pond5 is a stock media marketplace, where you can buy and sell stock media. One of the keystones of stock content is that it is short, so that it can be inserted into other projects with greater flexibility. We are unlikely to begin seeking longer videos because there is little demand for them in terms of stock content.
You do not need to, you can merely provide an option on a particular video for a longer clip Im sure a ton of the vidoes are longer but have been cut and edited so they can be submitted
That's a fair point. :)