Why did Nichiren criticise Zen?

Zen rejection of the role of voice and written teachings in Buddhist practice - constitutes a major difference between Zen and Nichiren Buddhism.
Zen asserts that the essence of Buddhism was transmitted from mind to mind, claiming that the Buddha’s enlightenment was telepathically transferred in this way to Mahakashyapa and then to each successive Zen patriarch.

Nichiren explains: “…the Zen school describes itself as a teaching transmitted apart from the sutras, and hence speaks with scorn of the Lotus Sutra.” WND1 p 1016
In his 20- years journey to study Buddhism at various temples, Nichiren described what he learnt from Zen masters:

“All the people throughout Japan have been led astray by the wild assertions […]
of the Zen school, which declares its teaching to be ‘a separate transmission
outside the sutras’… So believing, over the past more than twenty years
I have never ceased to cry out in a loud voice against these errors,
fearing neither the ruler of the nation nor the common people. WND 1 p 442

Nichiren questioned: how consistent is it for a teaching which rejects written or voiced words in its practice, to use words themselves - in order to convey its teaching (about rejecting words) :

“ …if one objects that we are hampering ourselves with doctrinal writings
and tying ourselves down with verbal explanations, and recommends a type of religious
practice that is apart from the teachings of the sutras,
then by what means are we to carry on the Buddha’s work
and make good causes in this saha world of ours?
Even the followers of Zen, who advocate these views, themselves make use of words
when instructing others.
In addition, when one is trying to convey an understanding of the Buddha
way, one cannot communicate the meaning if one sets aside words and phrases.” WND1 p 117

In response to his criticism to Zen and other schools, Nichiren and his followers were met with harsh treatment from the authorities of his time, which were supported by Zen and other influential Buddhist groups. Zen in particular was an appealing form of practice for the aristocracy and for the samurai class, and enjoyed support from the authorities at that time.

Nichiren mentioned that despite Zen masters animosity and hatred towards him, his aim was to help them see the depth of the Lotus Sutra’s teachings to save people from sufferings:

“Even in the case of ….the Zen priests…, all of whom bear me such hatred —
I admonish them because I want to help them,
and their hatred for me makes me pity them all the more”. WND1 p 608