User blog comment:LegerPrime/Duel-Element Dragon Mode/@comment-1776824-20160927042759

It's because they have no need to in light of the fact that they have modes that are stronger in their place. Natsu has been using Fire Dragon King Mode (in parts) since Avatar, whereas Gajeel didn't use Iron Shadow Dragon Mode in his fight against Bloodman because: A) he did not get a real opportunity to, and B) because he was going to go Dragon Force anyway. It was just writing for convenience on the part of there was no real point due to Dragon Force being on the horizon.

Rogue also has no need for White Shadow Dragon Mode because Sting and he nearly always fight together; if Rogue gets the mode, then Sting will need it as well. Its power also pails in comparison to Dragon Force. The only reason Future Rogue used it was just to show that there was another power, as Rogue of the present had already been revealed to use Dragon Force, and Natsu had activated his Lightning Fire Dragon Mode. It was responsive writing.

Wendy also has 100% no need of one. She can use Dragon Force. Why Natsu is using Fire Dragon King Mode instead of Dragon Force is beyond me. The power of Dragon Force is the power of a dragon itself. Fire Dragon King Mode's major power has been used up and it is no longer as strong as it was. But I'm not the author, and I don't write plot holes and contradictions. *shrugs*