1. He's charismatic enough to make the role memorable and has the star-power to sell tickets.

2. Comic book villains often deliver a triumphant final speech – no one else delivers monologues like Robert Downey; no one. He's top tier!

3. The characters of Iron Man and Dr. Doom are symmetrical in a thematic sense. Both are billionaire sycophants who seek to control the world around them. And they both wear masks — so there's that.

4. Comics are a medium that often rely on "heel turns", a term borrowed from wrestling that refers to the moment when a heroic figure turns into a "heel" or a villain. In essence this recasting is in keeping with tradition — especially if Doom ends up being a Tony Stark variant. [1]

   “Logic points to Downey’s Doctor Doom being from the same world that is home to the upcoming Fantastic Four: First Steps, which takes place in an alternate version of Earth within the vast multiverse. It is possible, while feeling oddly implausible, for Victor von Doom to look like Tony Stark on that alternate Earth.”

5. Robert Downey Jr. has the most starpower invested in the MCU; his brand as a celebrity is ultimately tied to this version of Iron Man. If he doesn't elevate The Fantastic Four's antagonist (Dr. Doom) in the same way: it will be Downey's bottom-line that takes the hit. He has the most to gain and the most to lose.

6. This decision adds a twist of freshness, even though the Marvel Cinematic Universe will still ultimately rely on nostalgia as the biggest selling factor —  this role-swapping could allow for more universe merging shenanigans in the long run. Perhaps the SONY "Venomverse" could finally find a home within the MCU. 

7. As an actor Robert Downey Jr. has come a long way since his induction into the Marvel movies via 'Iron Man', the original hit film by Jon Favreau that started this whole affair off. He's won an Oscar for his role in Oppenheimer, starred as Sherlock Holmes in two blockbuster movies, he attempted a reboot of the iconic character Dr. Doolittle, and he helped launch the Sony Pictures, Spider-Man spinoff franchise — so it's no wonder the studio has faith in Downey's ability to legitimize this new addition to the MCU.

8. No one really cares about "The Fantastic Four" as a concept. Sure, Silver Surfer and Galactus offer some cosmic appeal but the core team are relics of a bygone era: one of Stan Lee's earliest creations; intrinsically tied to the decade they were created in – 1960. Personally, I've never felt that "The Fantastic Four" were as easy to contemporize as some of the other legacy characters; Daredevil, Hulk, and Captain America, etc.

 Sidenote: Thor also has an outdated feeling in the comics, the movies solved that problem with witty banter and self deprecation.

Perhaps this upcoming film (Fantastic Four: First Steps) will go the same route in introducing “The Fantastic Four”, making them mostly comedic heroes, like “The Guardians of the Galaxy”. However, giving RDJ the task of supplying a "bankable" villain makes sense; if nothing else, there's an incredibly famous star of the MCU playing the bad-guy. Even if the marketing is lackluster, and the heroes end up forgettable, that's still a pretty good reason to see the movie — right?

Sources:

[1] https://www.hollywoodreporter.com/movies/movie-news/is-robert-downey-jr-avengers-doomsday-1235963482/#:~:text=Because%20Avengers%20directors%20the%20Russos,t%20be%20a%20Stark%20variant.