In the absence of quotes from the studio (I googled a whole bunch and found nothing!), here are all the titles of released movies, grouped by series:
- Iron Man
- The Incredible Hulk
- Thor: The Dark World
- Thor: Ragnarok
- Captain America: The First Avenger
- Captain America: The Winter Soldier
- Captain America: Civil War
- The Avengers
- Avengers: Age of Ultron
- Avengers: Infinity War
- Avengers: Endgame
- Guardians of the Galaxy
- Guardians of the Galaxy Vol. 2
- Doctor Strange
- Spider-Man: Homecoming
- Spider-Man: Far From Home
- Black Panther
- Captain Marvel
It looks like the first appearance of a character or team always gets titled with just their name, unless:
- There's already a popular 2002 movie from a different studio with that title (Spider-Man); or
- There's an unprecedented crossover movie coming out next year that you really want to market, and the movie you want to market it with (Captain America: The First Avenger) could use an alternative title for countries that aren't so keen on America
So far, all sequels from The Dark World onwards have had subtitles — except for Guardians of the Galaxy Vol. 2, and Ant-Man and the Wasp.
- Arguably, Ant-Man and the Wasp is the first movie featuring both heroes as a duo (because Hope wasn't the Wasp in Ant-Man), hence no subtitle.
- The team featured in the MCU Guardians movies is the team from the second volume of the Guardians of the Galaxy comic, so its title may be a reference to that.
Moving on to pure speculation, the subtitles for sequel movies in general might be following the idea of comics storylines having titles in their own right (Planet Hulk, Maximum Carnage, etc.)
They also might help to avoid the impression that each character's movies are a standalone series. Captain America: Civil War, for example, features a strong arc for Iron Man, and development for Black Widow, Wanda and Vision, alongside Cap's own story. Calling it Captain America 3 might have sat weirdly with the movie's actual content. (Of course, history is littered with smarter idiots than me speculating about the effect of titles on movie-goers.)
Phase 4 update
On 20th July 2019, Marvel announced two sequels and four title character debuts. Black Widow, The Eternals and Blade are titled with just their lead character/team's name, while Thor: Love and Thunder proves that the colon: subtitle pattern is alive and well.
However, the Doctor Strange sequel and Shang Chi debut swap colon for preposition and conjunction respectively with Doctor Strange in the Multiverse of Madness and Shang-Chi and the Legend of the Ten Rings, proving that even 11 years deep in the game, Marvel still has the power and the courage to innovate titles like no other studio. (No other studio at all.)