Hi Hooperdrivestheboat, and welcome.
One reason is that when you ferment the fruit in the primary the yeast is likely to be so active that much of the aromatic and flavor molecules will be blown out the fermenter and so lost. If you were simply making a wine that may be (may be) less of a problem but if you are making a mead then half the fermentables technically, must come from the honey so your volume of fruit will be reduced to begin with.
Another possible reason is that if you allow the fruit to macerate in alcohol (in the secondary) rather than in fruit juice (aka sugar water) in the primary, the alcohol is able to extract more of the color and flavor. A third reason is that honey can behave in the presence of yeast as far more acidic than you might expect because the honey has no chemical buffers. Fruit is often quite acidic and so the addition of fruit in the presence of honey may result in the pH dropping so low (the acidity becoming so high ) that the yeast will simply cease their work. There may be other reasons