Why isn't Gildarts one of the Ten Wizard Saints? He clearly is powerful. Makarov even said that Jura's (One of the 10 Wizard Saints) power was close to Gildarts, which means that Gildarts is more powerful than Jura.

So why isn't he a wizard saint?  Does he have to do something magnificent?  Or is he just not popular enough?
The Chairman of the Magic Council appoints the wizards to the Ten Wizard Saints. The Magic Council's decisions are not always objectively taken, and sometimes blatantly politically motivated. Many of the Magic Council members have a strong dislike for the Fairy Tail guild. It is likely they did not appoint Gildarts because they did not want two Fairy Tail wizards to hold that title. 

Another reason is that Gildarts travels a lot going on long missions, and rarely stays at his own guild for long, while the Magic Council probably did not want such a "nomadic" member on that elite group. Moreover, even if the Magic Council made him that offer, Gildarts himself may have turned it down, seeing how he returned the Fairy Tail guild Master's title saying that he is not suited for it.