Why can't everyone bend if they learned it from animals?

So it's been over 10 years since I watched TLoK and I was thinking about Wan's origin story, but having a generally hard time piecing it back together.

Wan's era was originally given bending from the lion turtles, but they eventually refused. It was said that afterwards, people learned how to bend again from observing animals (or the Moon for water)...

If it's simply a skill to be learned, why can't everyone do it?