After the first Avengers, we knew that Thanos will end up being the biggest villain of the MCU. However, after almost two entire phases, we knew nothing about the power of the Mad Titan.
And that is because Thanos sat on his throne and waited for others to do his work. What we knew was, among others, that he sent Loki and Ronan to get two of the Infinity Stones. However, if he was so strong and feared, why didn't he do it himself?
Why did he have to wait for so long and why did he decide in the last moment to do it himself?
The reason for this is very simple, according to director Joe Russo and writer Stephen McFeely. To get the stones, Thanos had to know the location of all of them. The reveal comes from the audio commentary accompanying Infinity War's digital download copy:
Stephen McFeely: “The catalyst for the whole movie was when Thanos… captured Nebula on his ship and rooted around in her brain and found out Gamora knew where the Soul Stone was.”
Joe Russo: “It’s the reason why he has not…”
McFeely: “Why he hasn’t gone after it before.”
Russo: “…Because the minute he does, the forces of the universe are gonna line up against him. If he doesn’t know where the Soul Stone is, then what is the point?” (via Screen Rant)
This confirms that Thanos knew the locations of all Infinity Stones, save for one. And when he captured Nebula, he was able to go after all of them.
Of course, there could be some issues with the timeline. And that is because we saw the Mad Titan donning the Gauntlet at the end of Age of Ultron (the MCU's 2015). However, by that time, he had already captured Nebula (MCU's 2014) and found out where the Souls Stone was.
Why did he wait so much?
Of course, as a master tactician, he waited for powerful beings to disappear - Odin, Ego, Ancient One fall into this category. However, the directors and writers of Infinity War didn't say this"¦