S01E03 of What If...? has characters voiced by the original (or usual, in the case of Hulk/Banner) actors:
- Fury: Samuel L. Jackson
- Hank Pym: Michael Douglas
- Coulson: Clark Gregg
- Rumlow: Frank Grillo
- Hawkeye: Jeremy Renner
- Banner: Mark Ruffalo
- Loki: Tom Hiddleston
- Sif: Jaimie Alexander
But there are several others that are voiced by different actors:
- Black Widow: Lake Bell not Scarlett Johansson
- Iron Man: Mick Wingert not Robert Downey Jr.
- Betty Ross: Stephanie Panisello not Lyv Tyler
- General Ross: Mike McGill not William Hurt
- Captain Marvel: Alexandra Daniels not Brie Larson
Has Disney or Marvel or anyone officially explained why this is?
It seems unlikely that Disney could not afford the original cast (and presumably actors like Samuel L. Jackson that were used would not have been cheap). Some actors, like RDJ, have presumably wrapped up their MCU work, but there are others like Johansson (potentially making the Black Widow film around the same time, and well before the current contract dispute) and Larson (presumably in the upcoming Captain Marvel 2) that are not. Much of the recording was done quite some time ago, and since it's voice work it seems unlikely that COVID-19 or scheduling would be issues.
While Captain Marvel is essentially a cameo in the episode, so might not justify the expense of Larson, Widow is a main character, and the voice change is jarring particularly when Black Widow and this episode have release dates so close together. Sif is also essentially a cameo, and yet is voiced by Alexander.