That spy satellite only had 22000 miles to travel, 22000 miles to transmit data back, and only had a couple of years to live.
Dawn has to travel hundreds of millions of miles, has multiple destinations to go to and orbit, and has to transmit its findings up to ~360 million miles back to earth.
Furthermore the imaging systems on Dawn need to take care of multiple roles. Spy satellites have but one job: image minute detail from just one focal distance: 22000 miles.
Rocket science ain’t easy
that probe has 1 mission... get out there, take clear pics of interesting things, and send it back
transmission distance is irrelevant.