There are other factors that can prevent forward splitter from doing a straddle split. For example the fact that adductors are also stretched due to lateral rotation in the center (side) split. Scar tissue and joint injury can be another.

Why are some people unable to do the splits?

Tight hamstrings and hip flexors are the top reasons why you can't do the splits. Doing splits is so much more than a cool parlor trick. Indeed, the ability to seamlessly slide into a split is a fantastic feat of flexibility and mobility, which is also why accomplishing the masterful move is so difficult.

Is it possible to never be able to do the splits?

Not everybody is able to do the splits, whether it's due to the bony anatomy of your pelvis or the amount of diligence needed to develop the right amount of flexibility. Everybody can make progress toward this goal, though — it's just going to take you more tha n a week to get there.