At this point in the movie, Emperor Palpatine sends a radio message to contact Darth Vader, and Vader instructs Admiral Piett to move out of the asteroid field so they can get a "clear signal". So one could perhaps deduce that Palpatine's signal was kind of broken up and blotchy, similar to bad phone reception in a tunnel or something.

Only when they move out of the field and Vader takes the call, it's still messed up and dropping in and out. I guess that's science fiction for you. They can travel faster than light at the drop of a hat, but still use non-error-correcting low bandwidth analogue radio signals.

Transcript

Captain Needa: {hologram} Do you see how ridiculously dense this field is? No one could survive that.
Darth Vader: No, Captain, they're alive.
Captain Needa: Um... <hack> <cough> <choke>...
Darth Vader: Your ship can survive an asteroid impact.
[SFX]: kerunch!
Darth Vader: See? Keep looking.
Captain Needa: <choke> <hack> <wheeze>...
{hologram fades}
Darth Vader: These warning chokes are terribly unsatisfying.
Admiral Piett: Um. Lord Vader?
Darth Vader: Admiral.
Admiral Piett: Interesting fact: This is the third time today that Emperor Palpatine has called to speak with you. That's 25% above—
Darth Vader: Set up the call.
Admiral Piett: Did you know a short range, broad spectrum radio source is flooding the asteroid field—
Darth Vader: Indeed.
Admiral Piett: It says "Hey, Darth Vader! Guess what? You suck!"
Admiral Piett: In over 6 million forms of communication.