Because now he knows the location of all of them and it's time to "Catch Em All ".
It turns out there’s a good reason for why Thanos waited so long, and Infinity War co-writer Stephen McFeely explains it below.
LRM Online pulled a quote from Stephen McFeely, who co-wrote the script with Christopher Markus, off of the Avengers: Infinity War commentary. (The film just hit Digital HD and comes to Blu-ray later this month.) Here’s what McFeely said about Thanos’s timetable:
“The catalyst for the whole movie was when Thanos figured out where the Soul Stone was because he captured Nebula on his ship [after she went after him at the end of Guardians of the Galaxy Vol. 2] and rooted around in her brain and found out that Gamora knew where the Soul Stone was…
Normally you want, you know, your inciting incident, the catalyst for this to maybe be on screen and maybe be first. But given how we wanted to approach the story we buried it to basically the midpoint. And, you know, again, it keeps you off balance, but it’s definitely there.”