Many have a custom not to say the Shir Shel Yom or Ein Kelokeinu on Tisha BeAv morning.
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Nit'ei Gavriel, Hilchot Bein HaMetzarim 62 fn 7 quotes an explanation from Responsa Zera Emet as to the common custom to omit shir shel yom and ein kelokeinu from shacharit and instead to recite them at mincha.
Essentially, one would be permitted to recite them at shacharit, despite the prohibition of learning Torah on Tisha B'Av, as they fall under the category of seder hayom (daily recitations).
However, the custom developed not to recite them, due to the strength of the aveilut in the morning (in the same way as the custom is not to wear tefillin nor sit on a regular chair until after midday).
So, they are not said in the morning owing to the prohibition of Torah study on Tisha B'Av, although they do not technically fall under the formal scope of the prohibition (which is why they can be recited in the afternoon).