Many (maybe most?) tinnitus cases develop after some partial hearing loss. The mechanism is not known, but one hypothesis is that the brain is "compensating" for frequencies it never hears anymore.
Hearing is logarithmic, the response is non-linear, ergo you may hear X000Hz but not hear X001Hz, yet hear X002Hz. X003Hz may be easier to hear, yet X004Hz may be more difficult than some. It's not a smooth response.
Take a look at some hearing response graphs to get an idea of how variable it can be across the frequency spectrum.
You may notice that I didn't say it was a universal, inescapable experience for those with tinnitus. The question was why there is any relationship at all between hearing loss and tinnitus, which does hold in a large number of cases.
