Performances under pressure determine the true greatness of a player in any format. Almost every good international cricketer who has played a sizable number of matches is capable of making hay when the sun shines. Whilst Amla has been a terrific Test batsman who has played some invaluable high-impact knocks under pressure, he has failed to make the same mark in ODIs and T20Is.
In fact, his chronic choking is the major reason why South Africa have not won a single trophy (WC, CT, WT20) during Amla's career, in spite of being heavy favorites regularly on paper. When your most prolific batsman goes missing under pressure almost every time, it becomes very difficult as a team to pull through. Of course, it doesn't help when your teammates have a documented history of losing the plot under pressure too.
Not only does he fail in tournaments, he also fails in big run chases - he will either get out cheaply, or he will play a typical inconsequential run-a-ball innings when the situation will demand a SR of 120+ etc., thus taking his team nowhere but padding up his stats in the process, before eventually throwing his wicket away because he doesn't have a second gear. He is a brilliant accumulator who can maintain a strike rate of 90 odd without much fuss, but when the require rate goes up to 8 and 9, he does not have the big shots to accelerate.
Average of 50 with a SR of 89 and 24 hundreds is a marvelous record, and his record in T20Is is more than decent as well, but the impact simply hasn't been there on the pitch. South Africa have not achieved (and will not achieve) anything of note in ODIs and Limited Overs cricket in general during his career, they have not won a single knockout game due to his batting, which is why he pales in comparison to the greats of the past. His stats are of a legend, but has merely been a good player, and is a shining example of why the South African trophy cabinet is empty in Limited Overs and will continue to collect dust.