Because 500-600 GB should be sufficient. Unless, if you already have a 500-600 GB and it's getting full.
*Nods*
Have an 80gb and i'm starting to run low on storage space. For a 1TB drive I find that rather useless unless you are using your PS3 like a computer DLing movies, music, and games. A 250GB would probably suffice but if you want a 500GB would work as well just be EXTREMELY careful. Do not I repeat DO NOT go for the 10k rpm drives as i've read they can cause your PS3 to heat up. Never read anything about the YLOD in relation to the increased RPM drives however i've heard of people reporting higher temps.