Because they spin/read faster than a 7200. The platter which stores data is different in how it works as the data cant be as compressed otherwise the head wouldnt be able to pick up the difference in magnetic strength as the disk spins. (at least i think thats why). For them to implement a 148gb raptor would be stupid cause very few people would buy one due to the fact it'd cost a friggin huge amount and you could just buy a 400gb maxtor instead.
It is possible to get a 300gb 10,000 rpm drive but its a SCSI which costs a bit more. They'll probably release a 147gb raptor later this year as the technology becomes older, but thats purely speculation.
A 147 GB Raptor would be just as easy (or hard) to make as a 74 GB Raptor. All they would need to do is add another platter or two and the associated arms and voice coil pickups. However, that adds more complexity, more chance or data loss, and a greater risk of mechanical failure. That's why enterprise drives like the Raptor are slower to increase drive capacities--it's because their customers are concerned (rightly so) about the integrity of their data.
Well then, i didnt think that a 10k HD would be able to spin so fast data would not be able to be read or that it might be loss i want a raptor sata, raid a couple together