Because of timing issues and crosstalk, parallel buses "max out" at a certain speed and it becomes nearly impossible to push past the barrier. There is simply too much interference and too much "adjusting" required to make everything work out just right. If parallel cables were made on wafer fabs, this could be avoided, perhaps, but the cost and practicality issues outweigh that.
Serial buses are freed from crosstalk and timing issues. All you need is super-fast controllers that can jam data through the wire. Today's semiconductors are more than cable of this, hence things like gigabit ethernet and such. Serial ATA just makes that possible between drive and motherboard. The drives themselves aren't going to change much, except for their controllers and physical interfaces.
Another side benefit of serial ATA besides speed is cleaner, neater, smaller cables. This will bring improved airflow and less need for cooling. Nice.
Mark