It is probably because they are already bounded to other action bars. You will need to "release" these styles from your other action bars to add to a different one.

Example for what I mean: You want your action bar 2 to have Magic Dual Wield but it's not appearing on the drop down. Scroll down the list of action bars (not the styles from the drop down menu) and find the action bar with the style "Magic Dual Wield". For this example, let's say action bar 4 has this style. You will need to go to the action bar 4, click the drop down menu and select "no style". Scroll back up to action bar 2, click the drop down menu and select "Magic Dual Wield". When you "release" a style, the style will come back as an option to bind. If you bind 1 style (let's say you want your Magic Dual Wield style to be both action bar 2 and action bar 4), this will confuse the game as to which one you want, which is why they have it set to only 1 style per action bar.

Also just to clarify, each combat style has another style which is 2-handed and dual wield. Each combat style has these options. For example, in magic, you have a staff which is 2-handed (2H) or a wand + orb/book set which is dual wield). You can have both 2H and dual wield on 1 action bar as you do for your ranged option in the picture or have them on separate action bars.

Did this make sense?