Ok, that's actually a tough question, and I find an answer from this french article
There is 3 kinds for these verbs :
dire and redire which conjugate 'vous -dites'
contredire, médire, prédire, dédire, interdire which conjugate 'vous -disez'
maudire which conjugate 'vous maudissez'
The original conjugation for 2 was -dites (e.g. vous contredites). Around the XVII e century, both conjugation could be found. The -disez was probably prefered due to 2 factors :
the little difference between the present (vous dites) and simple past (vous dîtes)
by analogy to the other verbs in -ire, which for the most, are conjugate '-isez' (e.g. vous lisez)
Dire probably stay the same due to its important use. According to the author, it's more difficult to change a word when it's used a lot (that's why common verb like être and avoir got such special conjugation, even in other languages).
Finally, for the verb maudire (curse), the explanation is close to the second kind of verb. But instead to conjugate it as other verb in -ire, it's probably the antonym bénir (bless) which was choose (vous bénissez).
Hope that's explain your question !