As Lonyo said.
There are other reasons not mentioned which, mechanically speaking, work in favor of the 2.5 inch format. It's easier to balance a smaller spinning mass, or I should say, it's likely to be less imbalanced compared to trying to spin a 3.5 inch disc at 10000 or higher RPM.
Apart from reduced power consumption, you're likely to get a bit less heat from the smaller drive also which is another bonus.
However, WD's move was not a big surprise. Even the last Quantum Atlas (before Maxtor and then Seagate) U320 SCSI drives, although 3.5 inch form factor only had 2.5 inch disc platters and I'm pretty sure that was before WD started doing anything like that with the raptors.
But to be honest, where I work we have over a hundred Dell Precision T7500 workstations all with WD 150/160GB 10K Raptor drives and whilst the speed impresses, and even the relative quietness of them, the longevity doesn't - they are by far the highest failing units.