The following contains spoilers for both Thor: Ragnarok and Avengers: Infinity War. You have been warned!
Marvel fans have been clamoring for it for months -- ever since it was first screened at D23 (then later, at San Diego Comic-Con). Where is the first trailer for Avengers: Infinity War? Audiences at those events saw footage from the pending Avengers movie. Those who couldn't attend want to know why they haven't seen it yet. And Thor: Ragnarok provided the answer.
You didn't know Thor (Chris Hemsworth) lost his eye yet.
The funny thing is, in the footage shown at both D23 and Comic-Con, Thor HAD both eyes. The footage opens on the Guardians of the Galaxy arriving at the site of a massive space battle. Thor lands on their windshield, and they bring him into their ship. Mantis uses her powers to revive the God of Thunder... and his face is intact.
So Marvel Studios likely used CGI trickery to protect the eye gouge from even those of us who saw the footage, so as to protect the vicious reveal of Thor's wound. If we all saw Avengers: Infinity War footage and it had Hemsworth wearing an eye patch, it's all anyone would have talked about heading into Thor: Ragnarok. By holding the footage, they are keeping one of the film's closest secrets. It's not a HUGE reveal. But it's a reveal, for sure, and I bet that factored into the delay of the Infinity War trailer.
The other reasons given by Joe and Anthony Russo are probably correct. They have said on record that you don't want to rush a trailer drop like this. They know that each and every frame of that trailer is going to be overanalyzed, and so it needs to be perfect. In fact, they shared this recently, to express their sentiments.
But now that we know about Thor, Marvel can release the hounds. Right? In a way, it would make sense to drop the first Avengers: Infinity War trailer on Star Wars: The Last Jedi. Where else are you going to get that big of an audience in a movie theater. The timing of it is right, also. Marvel doesn't want to wait until Black Panther in February. Though, Infinity War is one of those movies that doesn't have to release anything, and people will show up on May 4, 2018.
Be sure that you have our Upcoming Marvel Movies page bookmarked. Things could get really interesting, if the studio drops its Phase 4 titles in the near future.