Why bit rate is same in a lower frequency spectrum and higher spectrum?
If you are a bass and sing A 220 Hz and B 247 Hz at 120 beats per minute, you can sing data at a certain rate. 

If you are a soprano and sing two quarter notes A 880 Hz and B 988 Hz at the same BPM, your data rate isn't any higher.

If you sing more notes (higher bandwidth) you could communicate a more complex score (e.g. a higher data rate).  So bandwidth (range of notes) matters, not being a tenor or alto-soprano (spectral frequency).

Same at RF frequencies.