Partly tradition, partly the hardware and partly the software.

For computing devices data is moved around, operated and stored in bytes consisting of 8 bits. (although modern ones operate just as well in double bytes of 16 bits) So the byte became the default means of designating the fundamental data element. Therefore the size of "stuff" is conveniently expressed in the number of bytes.

For transmission, however, most modern communications and networks operate on a single line transmission. There are parallel communication methods but they tend to be local. But things like USB, wi-fi, internet have hardware that send data in a stream of bits. Therefore the speed of transmission is conveniently measured in bits per second