Answer: That's a good question, and simple words are my favorite kind.

The church modes in polyphony are really, as Knut Jeppeson puts it, the transition form between the old Gregorian modes and the modern major and minor. The needs of polyphony tended to encourage cadences in which the 7th degree of a mode was raised if not already a half-step from the tonic, and that sort of thing gradually reduced the distinctions between the modes.

But the Phrygian mode is special - the only real mode that begins with a half step - and if you want to write in a specific mode then you want to display its most obvious characteristic, which in this case is that half step above the tonic. So cadences in Phrygian typically have one voice moving downward by half step to the tonic. Anyone who hears it thinks: ah, that's Phrygian (well, maybe not anyone).

Now, if you also raise the seventh in another voice that would create an odd and technically dissonant interval in the cadence: a diminished third (for example, D# to F). Better just to leave the 7th degree natural, which also makes the special character of Phyrgian more evident. So it's customary to generally avoid raising the 7th in a Phrygian cadence.