What happened to the B: drive in Windows and why does the hard drive default to C?

Because back in the day of floppy disks, there were either two physical floppy drives (A: and B:), or just one physical floppy drive (A:) with one emulated (B:) so you could copy from disk to disk by exchanging disks every few hundred KB.

The A and B slots are very useful when you want to give a particular removable device the same drive letter each time it's inserted. Windows will never assign A or B to a device, but if you assign A or B to a device using Disk Manager, that drive letter will be assigned on future inserts.

I keep my source control database on a USB key so I can transfer it between multiple machines, and always assign it to B because I know that drive letter will be available on every machine. Finding this trick simplified my life greatly.