Charles Austin Miller

Answer: Here's a bit more detailed of an answer, after I had some more time to think about it. Remember that Anakin is still young, in his mid to late 20's approximate. He still had a ways to go till he reached the rank of Master on his own anyways, with the council being skeptical of him to begin with. His induction onto the Council was not something they wanted in the first place. Rather, it was Palpatine that appointed him as his representative on the Counsel.The Jedi had been growing to distrust Palpatine in this time and even Mace Windu stated that they could feel the Dark Side surrounds him. They didn't yet know he was secretly Sidious, but even still they had a growing concern about him and his ability to stay in office longer than intended as well as him reaching for more power. The Jedi allowed Anakin onto the Council because it was an opportunity for them... to use Anakin to spy on Palpatine. An assignment given to him off the record. They denied him the rank of Master for a number of reason. 1. His age. Again he's not even 30 yet and Obi-Wan even stated that to be on the Council at his age was unheard of. 2. They didn't trust him fully. He hadn't earned the respect and experience needed for the rank of Master yet. And there was the matter of Yoda sensing his future to be very clouded and unsure. Like Charlies said, his exceptionally high Midiclorian count and his unpredictability due to it was a high cause for concern. 3. It's likely they didn't intend to keep him on the Council. The war was close to ending, or so they hoped. The Separatist armies were dwindling and on the run. It looked like victory was a sure fire deal in the near future, as Count Dooku had been killed and they had traced the location of General Grievous. Once Grievous was dealt with, the war would pretty much be over at that point. They just feared that Palpatine might not relinquish his power at the end and needed someone close to him to keep an eye on him. Anakin was the only one among their ranks that could do that job due to his friendship with Palpatine. Which also that friendship was another red flag for them for Anakin. I believe that once the war was over and Palpatine finally removed from office now that things were settled, they did not intend to keep Anakin on the council any longer since he was an unwelcome semi-forced member in the first place. Allowing him to possibly earn his own way in the future and achieving the Rank of Master on his own right, if he could do it. So, basically, they didn't trust Anakin, he was too young, they needed him on the Council only temporarily, and intended to boot him off the Council and let him earn his own way back on his own after the dust had settled from the war. Just as we know... things didn't turn out that way.

Quantom X ★

Answer: He had not yet proven himself enough. Keep in mind he's still pretty young here, somewhere in his mid 20's. The Masters are older, more experienced and proven Jedi who are wise and have gained the respect. Also his closeness to the Chancellor made him a liability and they were not sure they could trust him to make decisions rationally when he was involved.

Quantom X ★

Answer: In "Star Wars I: The Phantom Menace," Qui-Gon Jinn discovers and reveals to the Jedi Council that 9-year-old Anakin Skywalker has the highest midichlorian reading ever recorded. Midichlorians, of course, are the microscopic lifeforms that allow Jedi and Sith alike to wield The Force. Anakin's midichlorians are off the chart, meaning that he has potential to be the strongest Jedi Master ever. Ever. That's why the Jedi Council approaches Anakin's training very, very cautiously. Rather than fast-tracking him to Jedi Master, the Council extends Anakin's apprenticeship to focus on his self-discipline, because they fear that this kid is a ticking bomb who could, potentially, flip to the Dark Side with catastrophic results (which is precisely what happens).

Charles Austin Miller

Answer: Anakin's rage drives him to make a deadly blunder, and he tries to attack Obi-Wan uphill; whereupon, Obi-Wan handily amputates both of Anakin's legs above the knee and amputates his left arm above the elbow.

Charles Austin Miller