Why did many Americans believe senator MacArthur accusations

That's a good question:

There could be many reasons why people believed senator MacArthur, but you wouldn't know exactly unless you surveyed the people that believed him. I don't want to waste your time to here's a few reasons why people may have believed him:

-He is a convincing person (some people have the ability to deceive others or get other to vote in their "favor").

-He had a lot of fans (his fans are more likely to support him no matter what).

-He had convincing evidence (if someone has relevant and good evidence / support to "back-up" what they're claiming, more people will believe them)