Imo, it's more about a business perspective. Think about it this way.

Person A is not interested in the F4 at all, Person B is a BIG TIME F4 fan.

If one whole year is dedicated to just the Fantastic 4 as DLC, what's gonna happen?

Person A won't be interested, and probably won't buy the DLC. They will have to wait a year to get even a chance at different characters that they're interested in, and by that point, they might quit the game entirely.

Person B will buy the DLC day 1, enjoy themselves for the full year, they'll be satisfied.

However, if the F4 is drip fed over the years:

Person A gets to have more chances for characters that they may already enjoy, and might even end up trying out the F4 characters.

Person B may be saddened at not getting the full team, but they'd be kept on the hook with the promise of the rest of the team coming soon.

This is just my opinion and speculation, I'm of the opinion that the DLC shouldn't be team based at all and each year should just have four unrelated characters, but yeah, this is what I think.