Why we shouldn’t recite the Naam using beads? Why should we do so through the breath alone?

If we recite it with the beads, our attention will remain in the beads and when the last bead appears, our attention will also get diverted to it. The tongue will remain busy in reciting the Naam and the Munn will find an opportunity to run away.

Manwa To Chahun Disha Phirey, Yeh To Sumrin Nahin  

What kind of a recitatation we can call it when the
Mind continues to wander in all the four directions.
The Mala (collection of beads attached with a thread) is related to the Mind whereas the recitation is related to the breath. The recitation through the breath means recitation through every pore of the body. In the breath itself lies our soul and the breath reaches every pore of our body. As such, it will mean the recitation with soul as well. That is why the enlightened souls, Sants have advised us to do recitation in the breath (Soundless with Surati).

Swans Swans Prabhu Sumir Le, Britha Swans Na Khoye  
Na Janey Kis Swans Mein, Aawan Hoye Na Hoye  

Don’t waste your breath. Use every breath in reciting
the Naam as none knows whether he will be able to take
the next breath or not. Guru Nanak Dev Ji says ;-

Swansa Di Mala Naal Sumiraan Tera Naam.  

I would recite your Naam with the beads of breath.
Remember! When the Munn escapes, the Surti as
well will escape and the recitation will become
purposeless. Another benefit of recitation through breath
is that it opens the Sushumna nerve. It will create
awakening and you will get absorbed in inward
meditation.

Jap Marey Ajapa Marey, Anhad Bhi Mari Jaye.  
Surti Samani Shabad Mein, Usko Kal Na Khaye.