Certified GKH Gardening Expert
If I had to guess, I would speculate that it is due to the age of the bulbs. Hyacinths need very specific conditions in order to bloom well year after year. These conditions are cool and moist in the spring and hot and really dry in the summer. The average garden just can't provide that.
At best, in most gardens, you will get 2-3 years of decent blooms before the hyacinths start to fade. Most people choose to simply replace them at this time.