Some Idori techniques are versatile in itself (as a hands-on technique) while others are way more appropriate if actually done seated. This happens to be the case for Ashi Dori.
Having said that, if you understand the principle you can always make it work whether you are standing up or sitting down.
Let's analyse Ashi Dori more deeply...
Forget about Idori for a second and imagine both you and your opponent are standing. Then simply 'copy paste' the techniques and here's what happens:
And yes of course... your oponent can always do something so the Idori versions are not without risk either. Having said that, Idori isn't built as selfdefense. It's Kata.
So where am I going with this?
I want you to understand that if one thing changes, everything changes.
Omote version: If you stand up, transform to grabbing the upper leg instead of behind the ankle. Do it with two hands and bump into your opponent with your shoulder.
Henka version: Since you go down primarily from standing up all the way to the ground, using your elbow is way more effective because that allows you to match the dropping action to your arm movement.
With this in mind, let's look carefully at the key points of Ashi Dori and discover how direction and positioning creates effective technique - or not.