The slower drives tend to be cheaper, need less power and generate less heat. No, they're not as fast, but for bulk data storage you usually don't need a fast drive anyway. When you're opening a 10MB word document it hardly matters if it takes you 0.1 seconds or 0.2 seconds. And with modern drives even the slow ones can play back several movies simultaneously without breaking a sweat.
If it was the OS drive then the poorer performance would be a lot more important, but for bulk data storage I personally prefer the slower drives.