The Curiosity rover has 12 cameras in total, but the main imaging cameras have 2MP sensors. That's what was said on Thursday by Malin Space Science Systems' Mike Ravine, who is the camera project manager for the mission.
He said, "These designs were proposed in 2004, and you don't get to propose one specification and then go off and develop something else. 2MP with 8GB of flash [memory] didn't sound too bad in 2004. But it doesn't compare well to what you get in an iPhone today."
It may not be just about that, though. After all, think about how much storage a high-resolution image takes. Then consider having to transmit that data back from Mars to Earth. We don't have subspace radio like the starship Enterprise did.
Despite that, the images seem perfectly all right to most laypeople - and scientists. The first 360 degree panorama sent back from Curiosity (shown), actually made up of 130 low-resolution (144 by 144-pixel) thumbnails combined, has elicited nothing less than oohs-and-aahs.